The Growth and Development of Insects

Eggs. Insect eggs differ in shape and color, and some are ornamented with ridges, spines, or other processes. Most insects lay their eggs in a situation where the young on hatching will have conditions suitable for development. Many lay their eggs in characteristic masses, and a few cover their eggs with a protective material of some sort. The eggs of some insects develop internally, and the young are born alive.

Insect eggs ordinarily develop only if they have been fertilized but some undergo parthenogenesis, that is, they develop without fertilization. Fertilization sometimes determines sex. In Hyme-noptera, for example, an unfertilized egg usually develops into a male and a fertilized egg into a female. Unfertilized eggs of most parthenogenetic insects develop into females, and in some species no males are known. A few Hymenoptera undergo polyembryony (a single egg develops into more than 1 young). This sometimes occurs in man, producing identical twins or triplets. In poly-embryonic insects, from 2 to more than 1000 young may develop from 1 egg.

Growth. The growth of an insect is accompanied by a series of molts, in which the exoskeleton (outer shell) is shed and renewed. Insects change in form as they grow, and the amount and character of this differ from group to group. This change is called metamorphosis.

The exoskeleton of insects is generally rather hard, and the extent to which it can stretch is limited. An insect cannot grow continuously but must shed the exoskeleton at intervals and replace it with a larger one. This shedding process is called molting, or ecdysis. Molting involves a shedding of the outer surface of the body, the linings of the tracheae, and the lining of the anterior and posterior parts of the alimentary canal. It begins with a splitting of the old exoskeleton, usually on the dorsal side of the head or thorax. In some cases (caterpillars, for example) the shed exoskeleton shrinks into a small irregular mass, but in others it retains the shape of the insect.

The stages between molts are called instars. The number of molts is generally 4 to 8, but may be as many as 20 in some insects. Molting usually stops when the adult stage is reached. Only a very few insects (like bristletails) continue to molt after becoming adult.

Metamorphosis. Successive instars differ not only in size but in other features as well. This change during growth (metamorphosis)

is relatively slight in some insects, very marked in others. There are 2 principal types of metamorphosis — simple and complete.

Simple Metamorphosis. In this type the wings (if present in the adult) develop externally during the early instars, compound eyes are present in the early instars if they are present in the adult, and there is no prolonged resting stage before the last molt. The immature instars of insects with this type of metamorphosis are called nymphs.

Nymphs usually resemble the adults except in size, body proportions, and the development of the wings; they generally live in the same habitat as the adult, and feed on the same foods. If the adults are wingless, the chief difference between nymphs and adults is in size. If the adult has wings, the wings are relatively small through the last nymphal instar, and expand to their adult size after the last molt.

Nymphs of mayflies, stoneflies, dragonflies, and damselflies differ from adults slightly more than in other insects with simple metamorphosis. They live in water and have gills, and when full-grown come to the surface of the water or crawl out of the water for their final molt.

Complete Metamorphosis. The eggs of insects with complete metamorphosis hatch into a wormlike stage called a larva. The larvae of insects vary in appearance: some have legs and others are legless and some lack a well-developed head. Larvae do not have compound eyes (but may have ocelli), and if the adult is winged the wings begin their development in the larval stage but develop internally. The larval stage lasts from a few to several instars, increasing in size and sometimes changing in color or other characters. After the molt of the last larval instar the insect changes to what is called a pupa. Pupae are usually inactive. They do not feed, and are sometimes enclosed in a protective covering, which may be a cocoon formed by the last larval instar before it molted or may be a puparium (formed of larval exo-skeleton).

Insect larvae vary considerably in form. Several terms (eruci-form, scarabaeiform, campodeiform, vermiform, elateriform, etc.) are used to describe them. Eruciform larvae are caterpillarlike, with a well-developed head, thoracic legs, and abdominal prolegs (see illus., p. 219); they occur in the Lepidoptera, Mecoptera, and some Hymenoptera (Symphyta). Scarabaeiform larvae are grublike, with thoracic legs but without abdominal prolegs, and are usually pale-colored and rather sluggish; they occur in certain Coleoptera (such as Scarabaeidae). Campodeiform larvae, which resemble diplurans in the family Campodeidae (see illus., p. 63), are elongate and somewhat flattened, with the antennae, cerci, and thoracic legs well developed, and are generally fairly active; they are found in the Neuroptera and many Coleoptera. Vermiform larvae are wormlike or maggotlike, without legs, and with or without a well-developed head; they occur in the Diptera, Sipho-naptera, and most Hymenoptera (Apocrita), and in a few insects in other orders. Elatenform larvae are elongate and cylindrical, hard-bodied, and short-legged; they occur in certain Coleoptera (like Elateridae).

Most insect pupae look somewhat like mummified adults, with the appendages free and visible. Such pupae are called exarate, and occur in most insects with complete metamorphosis except the Diptera and most Lepidoptera. Some pupae have the appendages

Stages in the development of a bug (simple metamorphosis). A, egg; B-F, nymphal instars; G, adult.

Stages in the development of a beetle (complete metamorphosis). A, egg; B-H, larval instars; I, pupa; J, adult.

glued to the body, and look much less like the adults. Such pupae are called obtect, and occur in the Lepidoptera and some Diptera (Nematocera). The pupa of many Diptera (Brachycera and Cyclorrhapha) is enclosed in a puparium, and is termed coarctate; puparia are oval and brownish, and look rather like the fecal pellet of a small rodent.

The different larval instars of some insects with complete metamorphosis are not of the same type: the 1st instar is campodeiform and the remaining instars scarabaeiform or vermiform. This type of development is called hyper metamorphosis, and occurs in some parasitic insects. The active 1st instar larva seeks out and enters a host and, once there, molts to a less active type of larva.

The larvae and adults of insects with complete metamorphosis are usually so different that one unfamiliar with their life history would not believe them to be the same insect. They often live in dissimilar habitats and feed on dissimilar foods. The transformation of an insect with complete metamorphosis is a remarkable phenomenon — something well worth following in the field or with caged individuals.

Changes in the Adult. Immediately after its molt to the adult stage an insect is soft-bodied and pale-colored. If the adult has wings, these are usually small immediately after the molt — about the same size as they were in the pupal (or last nymphal) instar — and must expand to their full adult size. This expansion may occur in a few minutes, or take a half hour or more. The coming-out of the adult at the final molt is called emergence. Darkening to the adult coloration generally takes place in a short time (an hour or less), but in some insects it may be a week or more before the adult has developed its full coloration. Some adults (such as Hymenoptera) that emerge from cocoons may undergo these changes before emerging from the cocoon. Others (a butterfly) have the coloration fully developed at the time of the final molt and the wings expand to their full size after emergence. Once the adult has expanded to its full size it does not grow any more, and (with rare exceptions) does not undergo any more molts.

Life History. Most insects in our area have a single generation a year. The adults are present for a limited time during some part of the year, and the winter is passed in a dormant state. Insects overwinter in different stages — some as eggs, some as nymphs, some as larvae, some as pupae, and some as adults. A period of dormancy at low temperature is often an essential feature of the life cycle. Many insects (particularly those occurring in the northern part of the country) will not complete their development unless exposed to low temperature.

Some insects regularly have 2 generations a year and others may have several — continuing to reproduce as long as weather conditions are favorable. A few require more than a year to complete their development. Many of the larger insects in northern areas take 2 or 3 years; the record holders are some of the periodical cicadas, which take 17 years.

Adults of most insects live only a short time, ordinarily from a few days to a few weeks. An overwintering adult lives several months, and the queens of some social insects can live several years. Many insects that are short-lived as adults do not feed in the adult stage.

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