Agriculture Reference
In-Depth Information
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 pre-
vents 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
