Development And Growth

Insects usually develop to adulthood through an egg and several pre-adult feeding stages (instars), either as nymphs or as larvae. Vivi parous insects, however, give rise directly to live nymphs or larvae and omit an egg stage.

Many insects reproduce sexually; in others, males may be extremely rare or are unknown and reproduction without a sexual phase (i.e. parthenogenesis) is normal. Members of some groups (e.g. many aphids) reproduce partheno-genetically and viviparously throughout the spring and summer but, after a sexual phase, lay eggs (the overwintering stage) prior to the onset of winter. Parthenogenetic reproduction in which only female offspring are produced is termed thelytokous parthenogenesis (thelytoky) - as found in aphids, certain Diptera and a few Coleoptera; parthenogenetic reproduction in which unfertilized eggs give rise only to males is termed arrhenotokous parthenogenesis (arrhenotoky) - as found in some scale insects, whiteflies and various Hymenoptera; partheno-genetic reproduction in which both sexes arise from unfertilized eggs is termed amphitokous parthenogenesis (amphitoky) - as found in a few Thysanoptera. Thysanoptera also exhibit arrhenotoky and thelytoky. In a few insects, notably certain gall midges, the adult and pupal stage is omitted from the life-cycle and larvae give rise parthenogenetically to further larvae; this process is termed larval paedogenesis. Pupal paedogenesis occurs where (again, as in certain gall midges) embryos arise within a 'pupa', which differs from a 'normal' pupa and is termed a hemi-pupa (see p. 173).

In the less advanced (hemimetabolous) insects, development through the pre-adult (nym-phal) stages to adulthood is gradual and usually involves only partial or incomplete metamorphosis, without a pupal stage (Fig. 7). In the more advanced (holometabolous) insects, metamor

Life Cycle Hemimetabolous Insects

Fig. 7 Life-cycle of a hemimetabolous insect, based on a psyllid - family Psyllidae (not to scale).

3rd instar nymphal stages

Fig. 7 Life-cycle of a hemimetabolous insect, based on a psyllid - family Psyllidae (not to scale).

Psyllid Life Cycle
Fig. 8 Life-cycle of a holometabolous insect, based on a butterfly - family Pieridae (not to scale).

phosis from the final larval instar to the adult occurs during a quiescent, non-feeding pupal stage (Fig. 8). In some groups, final-instar larvae enter a non-feeding prepupal phase, often overwintering as such and finally pupating in the spring.

Insect eggs vary considerably in appearance. For example, they may be spherical, oval, hemispherical, cigar-shaped, flask-shaped or sausage-shaped; they are sometimes flattened, fried-egg-like structures (e.g. Lepidoptera: Tortricidae). The outer, protective, waterproof shell (chorion) may be smooth or distinctly patterned (often reticulated or ribbed), and a distinct pore (micropyle) is sometimes visible. Eggs of phytophagous insects are often laid on host plants (or inserted within plant tissue) and are frequently cryptically coloured; they may be deposited singly or in small or large groups, and are sometimes protected by secretions, scales or body hairs from the egg-laying female. In some insects (e.g. cockroaches) the eggs are laid in protective sclerotized cases called oothecae.

When ready to hatch, the young insect usually bites or bursts its way out of the egg, sometimes with the aid of shell-bursting cuticular spines; first-instar larvae of some insects (e.g. certain hymenopterous endoparasitoids) have a distinct caudal process which functions as an egg-tooth. In some instances (e.g. as in the honey bee) the chorion may be dissolved away. Eggs of certain insects (e.g. Hemiptera: Miridae) have a distinct operculum (Fig. 9) which, at egg hatch, opens to allow the first-instar nymph to escape.

The hard external skeleton of an insect prevents steady growth. Instead, pre-adult insects develop through several moults (usually from four to ten), when the 'old' outer skeleton (i.e. the cuticle) is replaced by an initially flexible one, during a process known as ecdysis. The stages between ecdyses are known as instars.

Insect nymphs are usually structurally similar in appearance to the adult but lack wings. Second-instar (and or later-instar) nymphs of winged insects typically possess distinct external operculum

Fig. 9 Example of an insect egg, order Hemiptera: family Miridae.

wing pads (= wing buds), which increase proportionately in size at each moult until the fully winged adult (imago) stage is reached (see Fig.

Insect larvae are typically of quite different appearance from adults and, unlike nymphs, they lack compound eyes; larvae may, however, have one or more simple eyes (often called 'ocelli' but more strictly known as stemmata) on either side of the head. At each moult, the head capsule is replaced by a larger one, and reference to the width of the head capsule is often of value in distinguishing between instars as this (unlike body length) remains constant throughout the duration of any particular instar (see Fig. 10). In many insects, the appearance of some or all of the instars may be very different; nevertheless, specific descriptions tend to be based upon the most frequently and usually most readily observed, final-instar stage.

Insect larvae vary considerably in body form but may be divided generally into four main types (Fig. 11):

• apodous - legless, often maggot-like larvae;

• caiiipodeiforin - elongate, dorsoventrally flattened larvae with well-developed antennae and thoracic legs;

• eruciform - often caterpillar-like larvae with a more-or-less cylindrical body, well-developed thoracic legs and hump-like (or more obvious) false legs (prolegs or pseudopods) on the abdomen; such larvae sometimes have a sucker-like pseudopod on the hind-most abdominal segment (as in some chrysomelid beetle larvae);

Fig. 10 Relationship between the size of the body and the width of the head capsule during the later stages of growth of an insect larva - Acleris comariana, family Tortricidae: (a) penultimate instar; (b) newly moulted final instar; (c) fully grown final instar.

• scarabaeiform - larvae with a thick, fleshy (often C-shaped) body, well-developed head and thoracic legs but no abdominal prolegs.

The body hairs or setae of nymphs and larvae may arise from distinctive plates, pinacula, and tubercles or from wart-like verrucae. Details of these features are often useful for distinguishing between species, as are external features of the respiratory openings (spiracles) and, when present, the respiratory processes which bear the spiracles.

Insect pupae are of two main types (Fig. 12): exarate - some or all appendages (e.g. antennae, legs, mouthparts, wing buds) free, i.e. not fastened to the body; obtect - appendages fastened to the body.

The pupae of many Diptera develop within a sclerotized, barrel-like puparium (Fig. 13). The latter is formed from, and maintains features (such as characteristic respiratory processes) of, the cast-off skin of the final-instar larva. The pupae of many Lepidoptera often bear a distinctive cremaster (a series of hooks or spines, often borne on a distinctive cremastal


Fig. 10 Relationship between the size of the body and the width of the head capsule during the later stages of growth of an insect larva - Acleris comariana, family Tortricidae: (a) penultimate instar; (b) newly moulted final instar; (c) fully grown final instar.

Eruciform Larvae
Fig. 11 Types insect larvae: (a) apodous. e.g. fly maggot; (b) campodeiform, e.g. lacewing larva; (c) eruciform, e.g. moth caterpillar; (d) scarabaeiform, e.g. chafer grub.
Campodeiform Larvae
Fig. 12 Types of insect pupae: (a) exarate; (b) obtect.

outgrowth) at the tip of the abdomen. Features of the cremaster, which serves to attach the pupa to a silken pad or to strands of silk, can be helpful for distinguishing between species (see Fig. 283).

Eri Silk Moth Final Instar

anterior respiratory process

posterior respiratory process

Fig. 13 (a) Puparium of an anthomyiid fly; (b) cross-section to show pupa within.

Classification of the Class Insecta

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