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. See Ladybird Beetle Larva story for more information.

Aphids on perennial ryegrass. Aphids pierce the plants with needle-like mouthparts to suck the sap. Click for closer view

The needle-like mouthparts used for piercing plants and
sucking sap are visible beneath this aphid's head.

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?

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.

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 (see 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.

Close-ups of wingless aphids: Click for much closer view Click for much closer view

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.

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 .

Winged aphid, ready for takeoff.