Close-up view of the brilliantly iridescent blue eyes of a scallop in Augsburg’s marine aquaria
Many people know of scallops as a tasty seafood, but most have never seen a living scallop. Scallops are diverse, with over 300 species of scallops living on the ocean floor worldwide. They range from shallow waters to areas several hundred feet deep. Scallops, classified as bivalve mollusks, hide some amazing secrets. For one, about sixty primitive tiny bright blue eyes eyes reside in rows along a scallop’s mantle edge to detect motion, light and dark. A scallop can easily regrow any lost or injured eyes. Although these eyes may or may not produce clear images, the ability to sense an object moving with the speed of one of the scallop’s predators allows the scallop to save its skin (or to be scientifically correct, its shells) by either shutting immediately or swimming away.
Secondly, scallops possess an unusual trait which most other bivalves lack: the ability to swim. Scallops can propel themselves away from danger by contracting their powerful muscles and “clapping” their shells together, forcing water out through openings on both sides of their shell hinge. They can move forwards backwards, make turns, and right themselves in this fashion. Scallops swim particularly when faced with a predator (e.g., a seastar). Otherwise, if left relatively undisturbed, scallops are fairly sedentary creatures that lie on the seafloor as they feed by filtering microorganisms from the water.
Those tasty cylindrical or disk-shaped morsels of scallop meat found in seafood shops are the adductor muscles that in the living scallops make their unique swimming ability possible.
The distinctive shape of scallop shells (with fin-like extensions on each side of the hinge), is familiar as the logo for a well-known oil company
Scallops normally rest on the on the seafloor with shells partially open to allow water to be pumped past the gills for filter-feeding and gas exchange.
Numerous fleshy tentacles extend from the mantle, looking like menacing teeth in this photo. These tentacles have sensory roles, helping to alert the scallop to danger that might warrant clapping the shells shut for protection, and they serve as a sort of sieve to prevent large particles from entering the mantle cavity (within which the delicate gills are exposed).
(*Tridachia is an older outdated genus name for this animal)
The beautiful iridescent ruffles in the photo above might look like the leaves or petals of some exotic plant, but in fact they are the ruffled flaps of tissue (parapodia) that outline each side of the back of a two inch sea slug that lives in the waters of the Caribbean and Florida Keys. This animal is commonly known as the lettuce sea slug, so named because when the animal is at rest the parapodia make this snail without a shell look a lot like a small head of ruffled lettuce.
The lettuce sea slug sitting on a patch of the marine alga Bryopsis, which is one of the sources of the stolen chloroplasts. This sea slug spends much of its time simply sitting like this in a well-lit location, providing the chloroplasts with the light energy necessary for the manufacture of food
The resemblance of the lettuce sea slug’s parapodia to leaves is more than just a coincidence, since these structures, and indeed the entire surface of the animal, function somewhat like leaves. In fact, this sea slug carries out photosynthesis within its body! Like many of its relatives, this animal feeds by sucking the cytoplasm from the cells of marine algae, but this species and some other sea slugs don’t completely digest what they ingest.
The lettuce sea slug keeps the chloroplasts (the structures within the algal cells that carry out photosynthesis) intact and relocates them to its skin. There the stolen chloroplasts continue to function, using energy from sunlight to build energy-rich food molecules (sugars) from carbon dioxide and water, but rather than feeding the algal cells that made these chloroplasts, the sugar that is produced feeds the sea slug. Just as the leaves of a plant provide the plant with more surface area for the capture of sunlight, the parapodia of the lettuce sea slug provide more surface area for light capture.
Lettuce sea slug (the same individual as in the other photos) crawling on glass in Augsburg College’s coral reef aquarium (pink birdsnest coral in background)
These parapodia also increase surface area for gas transfer (oxygen uptake and carbon dioxide release), so they serve double duty, functioning somewhat like gills.
The lettuce sea slug is not alone in its habit of gaining extra functionality using cellular structures stolen from the organisms it eats. Some of its relatives (certain of the sea slugs known as nudibranchs) arm themselves with stolen nematocysts (tiny stinging structures produced by the cells of anemones, hydroids, and their relatives) from the cnidarians they eat! Somehow they manage to eat their prey without discharging all of the nematocysts, and they relocate some of these nematocysts to certain regions of their skin, where they continue to function. Many would-be predators of the nudibranchs are stung upon contact with the nudibranchs.
Closeup of ruffled frill of tissue along back of lettuce sea slug
More information, and many more photos can be found by clicking on the following links:
For further information on the lettuce sea slug, go to: http://www.seaslugforum.net/elyscris.htm. This page also has some wonderful photos showing the great variation in color within this species, and some really nice photos of tiny juvenile lettuce sea slugs in an aquarium where reproduction occurred. I highly recommend you take a look at this site, if only to see the beautiful lettuce sea slug photos.
For information on solar powered sea slugs in general, go to: http://www.seaslugforum.net/solarpow.htmw.htm
For more information on nudibranchs that arm themselves with stolen nematocysts, go to: http://www.seaslugforum.net/defcnid.htm
For a great deal more information on sea slugs in general, and many beautiful photos of colorful sea slugs, explore the pages of the Sea Slug Forum: http://www.seaslugforum.net/
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Head of tiny polychaete worm
Late in the evening around 10:00 PM on March 9, 2002, excitement had broken out in the sea urchin tank in Augsburg’s general biology lab. In the darkness of the night, a multitude of tiny creatures were swarming in the tank. At first it was difficult to tell what these small swimming creatures were. However, an overhead projector was nearby, and strong side illumination of the tank using the projector gave a clear view, revealing their identity. The small creatures were in fact small worms a centimeter or so in length, swimming about and presumably spawning.
These worms normally stay hidden in the sand and rocks, though they are occasionally seen crawling about on the walls of the aquarium, especially at night. This particular evening though the worms were all over the place, zipping about, and attracted to the light like insects to a porch light on a summer night. The next several evenings not a single swimming worm could be found, so it appears the night of March 9 was indeed somewhat special.
Polychaeta Swimming
Though members of the same phylum (Annelida, the segmented worms) as the earthworms in your backyard, these tiny marine worms are quite unlike anything you are likely to find in your yard. While earthworms and most of the other worms on land and in freshwater are oligochaete worms, most species of marine worms (including the little swarmers in our aquarium) are polychaete worms (Note: the primarily freshwater leeches belong to yet a third common class of annelid worms).”Oligo” means few, and oligochaete means “few setae” (few bristles). “Poly” means many, so polychaete means “many setae”. Indeed, as can be seen in the photo below, the little swarming worms are equipped with abundant bristles, much more so than the earthworms in your garden. Many polychaete worms also have much more distinct heads (often with well-developed jaws) than oligochaete worms, and many have conspicuous leg-like appendages called parapodia (also visible below…the setae are attached to the parapodia).
Closeup of one of the swimming worms. We don’t know the identity of this particular species, though it appears to have arrived in our aquaria along with a shipment of sea urchins from Florida. Several similar worms were present among the detritus in the urchin shipping water.
Most marine invertebrates (animals without backbones) and marine fish have planktonic larval stages that typically look very little like the adults. These planktonic larvae drift in the water currents for days, weeks, or months (depending on the species), feeding on planktonic algae or other planktonic organisms until they eventually undergo a metamorphosis to the adult body form. They then settle out of the plankton into suitable habitats (the few lucky survivors that did not get eaten or carried to unsuitable locations do anyway), where they take up a lifestyle more similar to the adults. To give their offspring a good start in life and help them to be better swept away in the water currents, many marine invertebrates and fish either climb to the highest spot in the area or swim to the water surface (as is the case with these small swimming worms in our aquarium) before releasing eggs or larvae.
Quite a few species of polychaetes engage in this sort of spawning behavior, and at certain times of the year swarms of spawning polychaete worms can be quite conspicuous at night in coastal areas. As is the case with many marine creatures, spawning is often coordinated with lunar cycles. In the Florida Keys and the Caribbean some polychaete worms are even bioluminescent! These worms, known as bioluminescent threadworms, can be quite conspicuous in nearshore waters in the Spring. Just after sunset, females of these worms swim to the surface where they flash a bright green light that attracts the males.
A diagram of an epitoke of a polychaete worm from the Biodidac collection at http://biodidac.bio.uottawa.ca/.
(BIODIDAC is a bank of digital resources for teaching biology)
As might be imagined, swimming about for the purpose of spawning subjects polychaete worms to increased danger from predators, but some species minimize this risk by producing a tail region that breaks loose and swims, carrying the gametes to the surface. These dismembered swimming worm tails are called epitokes, and they allow the worm itself to remain safely hidden while its sperm or eggs are carried up into the water column for fertilization and dispersal on the water currents to new sites .
Polychaete worms of many shapes, sizes, and lifestyles are extremely abundant and diverse in marine (saltwater) habitats. Some are free-living like the ones discussed here, while others such as the feather duster worms (which live in tubes that they construct) live much more sedentary lifestyles. Many sorts can be found in reef aquaria such as Augsburg’s coral reef aquaria, and many more types can be found in the wild.
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Close-up of feathers of male bay-breasted warbler
The weekend of May 12, 2002 was a big one in Minneapolis for migrating birds. Practically overnight, it seemed there were migrating birds everywhere, both in residential neighborhoods and in natural areas such as the Mississippi River Parkway. For example, during the course of an hour-long walk along the West River road about 1.5 miles south of Augsburg College on the evening of the 12th, my family and I spotted multiple individuals of at least 5 different species of warblers, not to mention an eagle (appeared to be a juvenile golden eagle) and a black crowned night heron.
The warblers were particularly conspicuous, and appeared to be highly distracted by the urgent need to find food. Normally wary of people, as they searched the grass, shrubs, and trees for insects many of these warblers seemed almost completely unaware of the people walking within feet of them. One of the most conspicuous was a male bay-breasted warbler searching for insects in a narrow strip of grass between the street and the bicycle path.
Male bay-breasted warbler foraging for insects along the West River Road, roughly 1.5 miles south of Augsburg College in Minneapolis
Bay breasted warblers are not common in this area, being seen only when migrating through to or from their breeding grounds to the north. Due to its apparent lack of concern about our presence, this bird attracted considerable attention from several sets of birdwatcher. We sat in the grass and watched and photographed this bird for about 15 minutes, during which time it came within a few feet of us on multiple occasions. Then, a bicycle came past and startled the bird, causing it to fly out in front of a passing car. Impact with the car killed the bird instantly.
Male bay breasted warbler moments after being killed by a car
After observing this bird for so long, seeing it killed abruptly was disturbing and disappointing. More importantly though, this event illustrates several more important points. Namely, migration is a difficult time for birds, and human activities can have a big impact on wild populations.
Many of our North American birds migrate. If asked why birds migrate, most people would probably say that they migrate south in the Fall to avoid the cold of northern winters, but that is mostly incorrect. The main reason for these migrations is due to food shortages during northern winters. Bird species that can find food all winter in the North tend to stay in the North all year round. Many species of seed eaters stay all year round, as do birds such as some of the woodpeckers that forage for dormant insects in tree bark. Even very small birds such as chickadees can survive harsh northern winters, despite their small size (which gives them a large surface area to volume ratio, meaning that they will tend to lose body heat extremely rapidly). The 50 or so species of warblers that nest in North America, on the other hand, are specialists at gleaning insects from vegetation, constantly flitting from twig to twig or branch to branch in their search for insects, and occasionally even grabbing an insect or two out of the air. The sorts of insects warblers eat are not available in the winter in the North, so they fly south in the Fall to spend the winter in more suitable habitats in the southern states, the islands of the Caribbean, Central America, and even South America. Birds such as swallows, swifts, and nighthawks are specialists at capturing flying insects out of the air, and hummingbirds feed on flower nectar and insects captured from flowers. These birds, along with many other species, migrate as well. Often migrating birds travel enormous distances, even crossing large bodies of water such as the Gulf of Mexico without stopping until they reach the distant shore – even tiny ruby-throated hummingbirds cross the Gulf of Mexico. There are some species of songbirds and shorebirds that even travel from Canada or Alaska to south Pacific Islands! It is important to remember that these birds are not seabirds….the ocean is an inhospitable place for these birds so they cannot stop to feed or rest until they reach land (in contrast, migrating arctic terns, which migrate from the arctic to the Antarctic and back every year are seabirds that have the potential of feeding along the way).
Such migrations are absolutely phenomenal feats, but they are undertaken because these migrations allow the birds to utilize the rich food resources (e.g. abundant insects) of the North during the breeding season even though these areas are not habitable for these particular bird species in the winter. These migrations are costly, however. Long migrations require phenomenal amounts of energy, which can leave the birds in an emaciated state. Not all individuals survive the rigors of migration, and those that do need to feed fast and furiously when they get the chance to regain lost body weight. In the Fall, there is a certain urgency because conditions are deteriorating in the North, so the birds cannot linger too late into the season. On the return trip to their breeding grounds in the Spring there is another sort of urgency, for there can be intense competition for breeding territories in the North… a bird that arrives too late might have difficulty establishing a breeding territory. Thus, these birds cannot dally any longer than necessary, but the danger of pushing North too soon is possible starvation should there be a late Spring cold snap. And if they live long enough, and some do, the birds have to face these problems year after year for a number of years.
And you thought YOUR life is difficult sometimes!
In the case of the bay-breasted warbler whose life was cut short by a car, this individual had possibly already made several round trips. This species nests throughout much of Canada and in a few slivers of the United States near the Canadian border, but spends its winters in northern South America, so it travels a significant amount each year. My guess is that the bird in these photos and many of the other warblers we saw that evening, were distracted by great hunger, resulting in riskier feeding behavior than they might normally exhibit. Furthermore, given that they were not local residents, they were less familiar with their surroundings than resident birds would be, and they were feeding in habitats that might not be their normal habitats (bay-breasted warblers normally nest in coniferous forests, for example).
Cars of course kill a lot of wildlife, not just migrating songbirds, but there were other hazards that evening for the migrating warblers. On our short walk we saw an inordinate amount of hunting behavior on the part of the local cats. Within a three block stretch within the local residential area, we observed two different cats stalking and nearly catching yellow-rumped warblers, and earlier in the day we observed a neighbor’s cat carrying a dead warbler in its mouth. Thus, not only do we humans reduce wildlife populations with our cars, but high predation from pet cats allowed to run free also reduces native bird populations.
Many birds, both migrants and otherwise, also die due to collisions with various man-made structures. Not the least of these hazards are large windows that birds attempt to fly right through, not noticing that there is glass blocking their path. To reduce such impacts images of hawks or falcons or models of owls are sometimes hung in or near large windows to scare off smaller flying birds. The photo shows the profile of a falcon placed in the large windows of the Lake Harriet bandshell in Minneapolis.
Though the above factors do indeed kill many birds, the effects of cars, cats, and man-made structures are probably small compared to a much more insidious effect of human activity….habitat destruction. Populations of many North American songbirds (and other wildlife) are declining, in part due to loss of breeding habitat in the United States, but also due to alteration of habitats in the bird’s tropical overwintering grounds due to human activities such as farming and deforestation. Indeed, habitat destruction is the most significant cause of the major declines in many animal and plant species that are occuring worldwide.
Fragmentation of forests into small patches also exposes forest dwelling birds to increased levels of nest parasitism by brown-headed cowbirds, which are birds of open country and forest edges. Cowbirds lay their eggs in the nests of other songbirds (removing a host species egg from the nest each time they lay an egg), who are tricked into raising the cowbird young. Often the young cowbirds are larger than the young of their adoptive parents, and the cowbird young compete for food and space and sometimes kick their nestmates out of the nest prematurely. This all can reduce the reproductive success by the host species.
Fruitbodies of Deflexula sp. fungus growing on a twig in Costa Rica (Family Pterulaceae, Phylum Basidiomycota)
I have made several trips to collect, culture and further study an interesting group of wood and leaf-decomposing fungi which are related to, but very different in appearance from, mushrooms. This group, the family Pterulaceae in the Phylum Basidiomycota (mushrooms, rusts, smuts, etc.), is primarily tropical, so my travels have taken me to Costa Rica, Puerto Rico, and the warmer parts of New Zealand.
The fungus shown above is a positively gravitropic, or downward growing, fungus in the genus Deflexula. This fungus may be found decomposing wood and leaves in the tropics and is only about 0.5 to 1.0 cm long. Deflexula (in culture) was the subject of an Augsburg biology student’s independent research project, focussing on nutrition, fruitbody development, and spore formation.
Pterula echo, a fungus from Bermuda studied and named by Augsburg biology professor Esther Mclaughlin and her husband David Mclaughlin
Another, rather similar, fungus from Bermuda is shown below growing upright (negatively gravitropically) in culture dishes. This fungus was the start of a multi-year study of members of the family. My husband and I named this isolate (Pterula echo) and published several papers on its development, cytology, classification, and reproduction. We now have more than 20 other isolates of Pterula in culture, from various locations.
Both the upright and downward-growing versions form spores on the surface of the slender white macroscopic fruitbodies. These fruitbodies look delicate or even fragile, but actually contain many thick-walled hyphae, making them tough and difficult to pull apart.
Recent studies at the University of Minnesota using molecular (DNA base sequence) evidence indicate that not only are Deflexula and Pterula closely related (i.e., they really do belong in the same family, despite opposite responses to gravity), but, surprisingly, they are related to the fungi cultured by the attine ants, an intriguing group of Western Hemisphere ants that collect plant material to feed “their” fungi which are in turn used as food by the colony.
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Nearly every year at about this time, Augsburg biology students get to experience one of the smells of the tropics, but this particular scent is unlikely to show up any time soon on the perfume counter. This is not one of the sweet smells we often associate with tropical flowers, but rather the powerful scent of rotting carrion!
The source of this smell, Amorphophallus konjac, is a member of the aroid family (Araceae), the same family as familiar plants such as Philodendron and jack in the pulpit. This plant from Southern China and Vietnam produces a powerful scent mimicking the odor of a rotting dead animal. This odor is irresistable to carrion-feeding insects, which are tricked into visiting the flowers and transporting the pollen by the false promise that the flowers are good places to obtain food and/or lay eggs. Most likely, the deceit is enhanced by the dark brownish purple rotting flesh color.
Amorphophallus is not alone in carrying out this sort of deceit. In fact, a variety of other plants have flowers that are less than pleasant smelling. Examples include the related skunk cabbage, and the unrelated wild ginger, both of which grow wild in North America.
This Amorphophallus plant spends its summers in Dr. Bill Capman’s backyard. During the growing season it produces one huge leaf, at least 3 feet tall and at least as wide, which vaguely resembles a small palm tree. It is quite attractive and exotic looking, and one would never guess its reproductive behavior by looking at it during the summer. In the fall, the plant goes dormant, and its huge tuber sits indoors in its pot in perfectly dry soil until the following May, when it goes back outside. Midway through this dry, dormant period the tuber sends up a rapidly growing flowering shoot, which develops in a matter of a very few weeks into the infloresence seen here. The blooming period last only a few days.
The water for this growth all comes from water stored in the tuber, and presumably, from the water produced by cellular respiration as the plant burns stored food in the tuber to provide the energy for this rapid growth.
The stench from this plant is truly impressive. So impressive that the plant is relegated to a position next to a fume hood during the peak of its blooming!
The huge “flower” of Amorphophallus is not actually single flower, but rather an inflorescence. In this photo and in the closeup below, the numerous small male flowers releasing dark pollen are above, and the lighter colored female flowers are below. (Note that the first photo at the top of this page was taken a few days earlier and the male flowers are white because they had not yet released pollen).
The amazing scent is produced from the large purplish structure that extends above the male flowers.
Amorphophallus konjac has lots of company within its own genus, it being just one of roughly 170 species of Amoprphophallus which grow in the old world tropics. While our Amorphophallus is a large plant, some of its relatives are downright enormous, with inflorescences exceeding 10 feet in height, and leaves up to 20 feet tall. When one of these huge species blooms in a botanical garden the event usually makes the international news.
What you are seeing here is the one single giant leaf that grows from the Amorphophallus tuber.
The following link gives a great deal of additional information on the genus Amorphophallus and its many species:
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The parents of the egg
Augsburg’s coral reef aquaria spawn approximately every two weeks through most of the year as long as they are well fed and there are no major disturbances in their tank. These frequent spawnings provide a supply of eggs that can be used to observe developing embryos in the lab or classroom. Small numbers of eggs can be removed using a small pipette, a process that is accompanied by much protest from the parent fish who attack the pipette to try to protect their clutch of eggs.
Three day old clownfish egg
If done with care, the eggs can be removed without damage, allowing living embryos to be observed, with clearly observable beating hearts and blood flowing through arteries, capillaries, and veins.
The egg shown in these photos was 3 days old at the time it was photographed. The large yellow structure speckled with black pigment-containing cells is the yolk sac, which contains the food used by the embryo. The embryos develop rapidly, changing noticeably each day, with the yolk sac steadily decreasing in size as fins, gills, mouth, and other body structures develop rapidly during a development period lasting a little more than a week.
Three day old clownfish egg. The “stalk” at the base of the egg is actually the tuft of adhesive filaments that held the egg onto the rock that served as the spawning site.
Though the eggs are tended carefully by the parents (the male parent especially), the larval fish that hatch from the eggs swim away and live a planktonic lifestyle before undergoing metamorphosis. Metamorphosis transforms them from larval fish that don’t look much like clownfish to miniature versions of the adults that begin to live a more sedentary adult lifestyle.
In aquaria, special provisions need to be made to ensure the survival of the young. To raise the young they must be removed shortly after hatching to a rearing tank and fed live marine rotifers as a first food. Rearing the young and rearing the rotifers to feed the young takes considerable time and effort, so thus far none of the young from this pair of clownfish have been raised.
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Winged aphid on blade of grass
Aphids are sap-feeding insects that can multiply rapidly when living on appropriate plant species (the many species of aphids have different food-plant preferences). When living conditions are good, most aphids reproduce parthenogenetically, the adult females giving birth to live female offspring without mating. The young reach reproductive age quickly, and population growth in these all-female colonies can be explosive. Though individual aphids are tiny, there can be considerable impact from the collective feeding activities of dense populations. Aphids are sometimes also responsible for the spread of plant diseases.
The aphids in these photos had colonized the grasses in experimental plant communities in our ecology lab during Fall 2002. These aphids in turn were being fed upon by ladybird beetles.
Also note the humped back where the wings attach. This area contains the musculature and other structures involved with flight. This humped back is lacking in the wingless aphids of the same species.
Sex, or no sex?
Aphids on perennial ryegrass. Aphids pierce the plants with needle-like mouthparts to suck the sap.
The idea of reproduction without sex might seem strange, but asexual reproduction makes sense for some organisms when conditions are good. Indeed, in addition to aphids and some other insects, a variety of other animals use a similar strategy, including the familiar water fleas (Daphnia) and numerous other small crustaceans, rotifers, and some other creatures (including a few vertebrates). And of course, asexual reproduction is commonplace among plants, fungi, protozoans, and bacteria. The explanation for this has to do with the relative costs and benefits of sexual vs. asexual reproduction under different circumstances.
If an individual is doing well, its success is testament to a good combination of genes that work well under prevailing circumstances. If living conditions are likely to be stable through the lifetimes of the offspring, genetically identical parthenogenetically produced offspring have a high probability of being as well adapted and successful as their parents, so asexual reproduction makes sense. Another big benefit favoring asexual reproduction is that parthenogenetic populations can grow faster than sexual populations since every individual is a female giving birth to offspring.
The needle-like mouthparts used for piercing plants and sucking sap are visible beneath this aphid’s head.
However, conditions might be different in the future, and different gene combinations might be necessary for success. Given this unpredictability, most organisms gamble for the future success of their descendants by reproducing sexually (and even bacteria have means of exchanging genes, though in bacteria this is not called sex), because the mixing of genes through mating produces genetically variable offspring. Though many of these offspring might be poorly adapted to the unpredictable conditions, there is a chance that at least a few in a genetically variable group will have “winning” gene combinations. This is analogous to buying 100 lottery tickets all with different numbers vs. buying 100 tickets all with the same number. If the winning combination of digits cannot be predicted in advance, 100 tickets with different numbers clearly give a much greater chance of stumbling onto the winning combination. Actually, even 50 or fewer tickets with different numbers would give a better chance of winning than 100 tickets all with the same number (and this is the better analogy, since sexual organisms tend to reproduce more slowly because the males do not give birth to young or lay eggs).
Evolutionary success is all about passing one’s genes on to future generations, and doing so better than other individuals. Indeed,”evolutionary fitness” (the “bottom line” in evolution, and the driving force shaping evolutionary change by natural selection) is defined as an individual’s or genotype’s probable genetic contribution to future generations. All currently living organisms are descended from ancestors that produced successful offspring. We don’t encounter descendants from individuals whose offspring were unsuccessful, because they left no descendants. Even among the successful reproducers, the genes from individuals with fewer successful offspring tend to be replaced over time by the genes from those with greater reproductive success. Though the production of males decreases a genetic line’s potential for increase (rate of increase is roughly cut in half, assuming an equal sex ratio) compared to a parthenogenetic, all-female genetic line, the benefits of sexual reproduction can be considerable. Since genetic variability increases the odds of having at least a few offspring survive and reproduce, most species reproduce sexually at least some of the time.
Most totally asexual populations(or asexual genotypes within a mixed population with both sexual and asexual individuals) would be expected to die out eventually because they are less able to adapt to changing conditions, which is why sexual reproduction is the norm for most species. In this context it is interesting to consider that the human obsession with the opposite sex (including such aspects of our society as the perfume and makeup industry) is a spin-off of the long-term evolutionary benefits of producing genetically variable offspring!
In the case of aphids, males are normally only produced at certain times of the year, such as the end of the growing season when living conditions are deteriorating. Sexual reproduction results in genetically variable eggs that survive the hard times (winter, for example) and hatch out as a new generation of parthenogenetic females when conditions improve.
Patterns of insect development
Some insects (such as beetles, flies, butterflies, moths, and others) have complete metamorphosis, in which the immature stages are larvae that look nothing like the adults Ladybird Beetle Larva story for more information), but aphids have incomplete metamorphosis.
Like all insects, aphids go through a series of molts, 
shedding their exoskeletons multiple times as they grow and mature, but
immature aphids have the same basic body plan as the adults. Note the similar body forms of the smaller juveniles and the
large wingless adult in the top photo at right.
Other insects with incomplete metamorphosis include cockroaches, grasshoppers, crickets, silverfish, and many others, all of which have immature stages that look like the adults and often have similar lifestyles to the adults.
Why don’t all insects have wings?
Whether an insect species has complete metamorphosis or incomplete metamorphosis, only adult insects have wings. Thus, despite what many people say, tiny flies buzzing around your head in the summer are not baby flies (baby flies are maggots or other worm-like larvae), and small bees visiting flowers are not baby bees (baby bees are grub-like creatures). These are simply adults of species that are very small.
If an insect has wings, it is definitely an adult, but it does not automatically follow that an insect without wings is an immature insect. Some entire taxa of insects lack wings (silverfish and fleas, for example), and some members of otherwise winged groups sometimes lack wings even as adults.In the case of aphids, most individuals are wingless (see photos above), but under certain circumstances or at certain times of the year, some individuals develop into adults with wings. Tiny wingless aphids cannot travel far by walking, making dispersal to new plants and new habitats difficult. However, wings allow some individuals to disperse to new plants where they can found new colonies. Tiny winged aphids sometimes even migrate long distances (hundreds of miles), by “hitching rides” on air currents (some species do this routinely).
Why don’t all aphids have wings? Wings are metabolically expensive, both to grow and to use (and the cost of growing wings also includes the associated musculature and other support structures…in the photos on this page, notice the humped thorax were the wings are attached, and compare to the thorax of a wingless aphid). If an insect devotes energy and resources to wings that insect has less energy and resources for other things such as reproduction. All else being equal, a wingless individual should be able to reproduce faster than a winged one. Thus, if living conditions are good it sometimes makes sense to be wingless.
Winged aphid, ready for takeoff
Despite the cost of wings, adults of most insect species have them anyway. Powered flight is only found in a few living taxa… the insects, birds, and bats (and a few species of South American fish that can fly short distances), and is one of the truly remarkable adaptations of the insects. Indeed, the ability to fly is likely a large part of the reason why insects have been so successful (see discussion of insect diversity in Ladybird Beetle Larva story).
One of the probable benefits of insect flight is that it helps insects evade predators, but as mentioned above, a possibly much greater benefit is that flight allows insects to disperse to new habitats. This allows them to exploit resources more effectively, and helps prevent total extinction of populations or genetic lines when environmental changes make conditions inhospitable in a local area.
Dispersal in space and time
As described above, when living conditions are good aphids maximize their reproductive rates by eliminating both wings and males. Wings and males are only produced under certain special circumstances when their benefits outweigh their costs. Wings and males have not been eliminated altogether because wings increase the ability to disperse to new habitats (thus avoiding total extinction should some disaster strike a given location), and males (and the genetic diversity that comes from sexual reproduction) increase the chances of “dispersing” into the unpredictable future .
Links
Here are a few web links with more photos and information on aphid life cycles. Many more links can be found in a Google search.
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Ladybird beetles are among the most familiar and recognizable of insects, but most people either don’t notice their larvae or don’t realize what they are when they see them. The larvae in fact are very common, spending their time crawling around on plants as they search for small insects such as aphids. Ladybird beetle larvae are often as colorful as the adults, with ferocious-looking spikes of several colors covering their bodies.
Ladybird beetle adult
The larva in this month’s photo was one of a number of larval and adult ladybird beetles that were found in our ecology lab in Fall 2002. They were feeding on aphids that had colonized the perennial ryegrass in experimental plant communities set up as part of student research projects.
Ladybird beetles are well known for their appetites for soft-bodied insects such as aphids, and they are often introduced to gardens, agricultural plantings, or greenhouses for insect control. Aphids are sap-feeding insects that can multiply rapidly when living on appropriate plant species (the many species of aphids have different food-plant preferences). Though individual aphids are tiny, there can be considerable impact from the collective feeding activities of dense populations.
Aphids on perennial ryegrass. Aphids pierce the
plants with needle-like mouthparts to suck the sap
Though ladybird beetle adults and larvae are both predaceous aphid eaters, in most insect species the majority of an insect’s feeding takes place in the juvenile stages (indeed, in some insect species such as mayflies the adults do no feeding at all). While adults need energy and resources to support basic metabolism, activity, and reproduction, the immature stages are where nearly all body growth takes place, and this requires a great deal of food. Aside from the production of eggs (or sperm), little if any growth occurs in adult insects.
Beetles have complete metamorphosis (insects with this developmental system are called holometabolous insects). Such insects go through a series of larval stages that look nothing like the adult, molting (shedding) their exoskeletons between each stage. Their exoskeletons increase in size by quantum leaps with each molt. The last larval molt produces the pupal stage during which metamorphosis to the adult body form takes place. During the pupal stage, nearly all body tissues and cells break down to a rich chemical soup. This gooey, self-digested larval body provides the building materials and fuel for the growth of a completely new body that develops rapidly from small clusters of cells that remained intact. With one more molting, the newly formed, completely remodeled adult body emerges from the pupal “skin”. After that, the insect will never molt again.
Ladybird beetle larva consuming an aphid on
a blade of grass.These larvae are voraceous aphid eaters.
Not all insects have complete metamorphosis (aphids do not, for example…immature aphids look like miniature versions of the adults), but complete metamorphosis (holometabolous development) is utilized by some of the most abundant and successful insect groups. In addition to the beetles, other holometabolous insects are flies, butterflies, moths, fleas, ants, bees, wasps, and some others.
Though adult and larval ladybird beetles exploit the same food resources, the holometabolous developmental system often allows juveniles to exploit different food resources from the adults, avoiding competition with the adults, and allowing a greater diversity of resources and niches to be exploited by the species overall. For example, most caterpillars feed on leaves, while adult butterflies and moths drink flower nectar. Larval mosquitoes feed on microorganisms filtered from the water, while adult female mosquitoes feed on blood. Corn rootworm beetle larvae feed on roots, while the adults feed on aboveground plant parts such as floral structures and leaves.
Student research project that was unintentionally colonized by aphids and ladybird beetles (this project investigated the effects of trampling on plant community structure).
When one considers that beetles are by a wide margin the most diverse group of animals on earth, and many of the other insect groups with complete metamorphosis are also quite diverse, it is quickly apparent that this seemingly strange developmental system is anything but rare. According to some estimates, the collective species diversity of just the beetles, flies, butterflies, moths, wasps, bees, and ants is around 740,000 species, with many more species yet to be discovered (Rupert, et al, 2003).
This is an enormous number, but just how big is this? To put this into perspective, consider that the beetles and other holometabolous insects make up about 80% of total insect diversity. Since the insects overall are twice as diverse as all other groups of animals combined, holometabolous insects (and especially the beetles) make up a majority of the Earth’s animal species diversity.
Holometabolous insects seem utterly bizarre from our human perspective. Indeed, self-digestion of the larval body and creation of an entirely new body from scratch seems like fantasy from a science fiction movie, and the ladybird beetle larva shown below looks like it could indeed be some sort of alien from another planet. However, holometabolous insects greatly outnumber us by any measure, be it species diversity, overall biomass, or simply sheer number of individuals. By this line of reasoning, these are actually the most ordinary of creatures. Perhaps we are the creatures who should be considered strange?
Links
A Google search for “ladybird beetle”, “coccinellid”, or “coleoptera” yields a great abundance of links to interesting and informative web sites dealing with ladybird beetles or beetles in general. Give it a try and explore!
Here are a few with information on ladybird beetles, including diagrams and some photos of the eggs and pupae:
http://ohioline.osu.edu/hyg-fact/2000/2002.html
Reference Cited
Ruppert, E.E., Fox, R.S., Barnes, R.D. 2003. Invertebrate Zoology: A Functional Evolutionary Approach. Thomson-Brooks/Cole, USA.
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A monarch butterfly in flight.
Most people are almost completely unaware of the tremendous diversity of insects that can be found even in a typical city backyard, and indeed, even if an insect is noticed it will likely take an expert to identify it to the species level. But butterflies are an exception. Most North American butterflies can be identified relatively easily by anyone with a good butterfly book, and some of the butterflies are exceedingly conspicuous insects recognized readily by the novice.
As butterflies go, there are few as well known as the monarch. Monarchs are well known not only because of their conspicuous appearance, but also because of their dramatic yearly migrations. In the eastern part of North America, the entire monarch population migrates to a small mountainous region in Mexico where they spend the winter (in the western part of North America the monarchs migrate to coastal regions in the general vicinity of Pacific Grove, California, near Monterrey Bay).
A monarch butterfly feeding on the nectar of cup plant flowers. Though butterfly caterpillars have chewing mouthparts for feeding on leaves or other plant parts, adults have mouthparts specialized for feeding on nectar.
Many people grow milkweed plants in their gardens specifically to attract monarch butterflies, which lay their eggs only on the various species of milkweeds. Milkweeds are toxic to most insects and other animals, but the monarchs are able to sequester the toxins in their wings without harm to themselves. In doing this they become toxic to predators, and their bold coloration serves as a warning to birds that the monarchs are best avoided.
The butterflies in these photos were photographed shortly before the Fall migration in September 2007 near Lake Hiawatha in Minneapolis. Though monarchs lay their eggs only on milkweeds, the leaves of which are the only food the larvae will eat, the adults feed on the nectar of a wide variety of flowers. By Lake Hiawatha that day they were actively feeding at the flowers of wild sunflowers (Helianthus spp.) and cup plants (Silphium perfoliatum, which are the flowers in these photos).
Why such large wings?
Butterflies in general have among the largest wings of any insects, and monarchs are large even by butterfly standards. Though butterflies look delicate, they actually are very strong flyers, and the size of their wings surely plays a role.
Aerial plankton vs. strong directed flight:
The flying monarch above has a black spot on the hind wing, which indicates this is a male. The black spot is a scent gland used in courtship
Many insects disperse long distances by flight, but most small insects are at the mercy of the wind currents once they are high in the air. Most small insects control their long-distance movements by choosing to fly up into, or drop out of, the air currents at specific times and places, but once they are high in the air they have little control over where they go (they do have control closer to the ground and in the vicinity of vegetation of course). These small insects high up in the air are often referred to as aerial plankton because they are at the mercy of the wind patterns much the way planktonic creatures in the ocean are at the mercy of the ocean currents. A great many insects disperse this way, and the air is filled with insects in the summer, up to fairly high altitudes. The variety and abundance of larger animals that specialize in harvesting this airborne bounty (including swallows, martins, swifts, nighthawks, and bats, all of which specialize in feeding on flying insects captured on the wing) is testament to the great abundance of flying insects in the air.
But large butterflies like monarchs (and also some other large insects such as dragonflies, large moths, and some others) are different. Their flight is strong enough that they can more directly control where they go, which of course is essential if they are to congregate in specific overwintering sites each year. Interestingly, in addition to the well-known migrating populations of monarch butterflies, there are also other non-migratory populations of monarchs in tropical regions and on some tropical islands. Recent research has found that monarch wings in the migratory populations are significantly larger than in the non-migratory populations, which lends support to the idea that larger wings are important for strong, long-distance flight.
See this article for more about this very interesting study:
‘Supersized’ monarch butterflies evolved to fly far
And a few more interesting articles on monarchs:
This article sheds some light on how migrating monarchs navigate:
Butterfly ‘GPS’ found in antennae
And this article discusses threats to monarchs due to illegal logging in their overwintering area in Mexico:
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