LIFE CYCLES OF INSECTS
Distinctive of the arthropods is their variety of immature forms. As an insect grows, it passes through a series of maturation phases, and each phase can look quite different from the previous or subsequent one, as demonstrated in Figure 1. Over millions of years of evolution, insects have developed three patterns of growth. The first and simplest is ametabolous (“without change”) metamorphosis, where the eggs yield immature forms that look like smaller forms of the adults. Eventually, these juveniles develop in size and mature sexually but otherwise undergo little structural change. This type of metamorphosis is limited to more primitive wingless insects (Apterygota). The second type of metamorphosis is paurometabolous, or gradual, metamorphosis. The hatchlings emerge in a form called a nymph, which generally resembles a wingless version of the adult of the species. The nymphs and adults will occupy the same habitat and exploit the same food sources. Nymphs grow by moulting (shed ding their skin), and each successive moult produces a new instar or growth phase. As the nymph passes through each instar, it increasingly resembles the adult form and eventually develops wings. Different species pass through specific numbers of instars, and this can be useful in identifying immature forms. Cockroaches (Blattaria) and various predatory bugs (Hemiptera), for example, develop this way. Holometabolous (or complete) metamorphosis is the third type of arthropod growth, and it is the most complex format of the three. The adult lays an egg (oviposits) or deposits a larva (larvaposits) onto a food source. The larvae (plural) start eating or hatch from the egg and then begin eating immediately and increase in size by molting through instars. The larval form is very different from the adult form, both in appearance and in its habitat. At the end of the instars, however, the larvae transition into an inactive phase, called the pupal stage. The pupa is a hardened outer shell or skin that protects the larva while it undergoes its final growth stage to the adult form. Butterflies (Lepidoptera) are a common example of holometabolous insects as they change from caterpillar into cocoon to their final, colorful adult form. There are several types of pupa in holometabolous insects, but the type most frequently encountered by forensic entomologists is that evidenced in flies (Diptera), the puparium. The puparium is the hardened skin of the last larval instar and tends to be darker than the normal larval skin (Resh and Carde, 2003). Depending on the species of insect, the time it takes to go from egg to adult varies greatly: Some insects may have a few or many generations in 1 year. The weather, environment, season, food (abundance or lack), rainfall, humidity, and other such factors all can affect the timing of insect reproduction. In the case of necrophilous insects (“dead loving,” or those associated with decomposition), many other factors, such as location (indoors, outdoors, on land, in water, etc.), shade, slope, and where the body lays (on soil, cement, in a tree, in an attic, etc.), can have an influence on the number and timing of successive generations. Necrophilous insects are very sensitive to chemical changes in a dead body and can detect even the slightest hint of decomposition, sometimes within minutes of death. The chemicals are by-products of the decomposition process and signal to the insect that a new food source is available. As the body decays, the signals it sends out change and communicate “food” to the different species that inhabit the body at different times and conditions. Dermestid beetles, for example, prefer dry flesh and won’t colonize a body until the tissues are no longer wet or even moist; by that time, the odors and chemicals coming off the remains are very different from those emitted, say, 2 weeks prior. The habitat of a decomposing body is a finely tuned environment, and insects have evolved to make the most of each stage of decay.
FIGURE 1 Insects develop through various life stages, depending on the type of metamorphosis they experience. Ametabolous insects have immature forms that appear to be small adults. Paurometabolous insects emerge from hatching into a nymph form, which progresses to adult through a series of moltings. Holometabolous insects develop from eggs into larva, which then go through a separate growth stage to reach adult form. The caterpillar spinning a cocoon and emerging as a butterfly is a common example, as is the housefly shown here.
