Introduction - Pests of Ornamental Trees, Shrubs and Flowers - page 3

distinguish between species (as in agromyzid leaf
miners).
Female insects possess a pair of ovaries, subdivided
into several egg-forming filaments called ovarioles. The
ovaries enter a median oviduct and this opens to the
outside on the ninth abdominal segment. Many insects
have a protrusible egg-laying tube, called an ovipositor.
The male reproductive system includes a pair of testes
and associated ducts which lead to a seminal vesicle in
which sperm is stored prior to mating. The male
genitalia may include chitinized structures, such as
claspers which help to grasp the female during
copulation. Examination of the male or female genitalia
is often essential for distinguishing between closely
related species.
Sexual reproduction is common in insects, but in
certain groups fertilized eggs produce only female
offspring and males are reared only from unfertilized
ones. In other cases, male production may be wanting or
extremely rare and parthenogenesis (reproduction
without a sexual phase) is the rule.
Although some insects are viviparous (giving birth to
active young), most lay eggs. A few, such as aphids,
reproduce viviparously by parthenogenesis in spring
and summer but may produce eggs in the autumn (after
a sexual phase). Insect eggs have a waterproof shell.
Many are capable of withstanding severe winter
conditions on tree bark or shoots, and are the means
whereby many insects survive from one year to the next.
Insects normally grow only during the period of
pre-adult development, as nymphs or larvae, their outer
cuticular skin being moulted and replaced between each

successive growth stage or instar. The most primitive
insects (subclass Apterygota) have wingless adults,
and their eggs hatch into nymphs that are essentially
similar to adults but smaller and sexually immature.
The more advanced, winged or secondarily wingless,
insects (subclass Pterygota) develop in one of two ways.
In some, there is a succession of nymphal stages
in which wings (when present in the adult) typically
develop as external wing buds that become fully
formed and functional once the adult stage is reached. In
such insects, nymphs and adults are frequently of
similar appearance (apart from wing buds or wings),
and often share the same feeding habits. This type of
development, in which metamorphosis is incomplete, is
termed hemimetabolous. The most advanced insects
show complete metamorphosis, development (termed
holometabolous) including several larval instars of
quite different structure and habit from the adult. Here,
wings develop internally and the transformation from
larval to adult form occurs during a quiescent, non-
feeding pupal stage. Insect larvae are of various
kinds. Some, commonly called caterpillars, have three
pairs of jointed thoracic legs (true legs) and a number of
fleshy, false legs (prolegs) on the abdomen. Many
butterfly, moth and sawfly larvae are of this type.
Unlike sawfly larvae, those of butterflies and moths
are usually provided with small chitinous hooks
known as crotchets. Some larvae, including many
beetle grubs, possess well-developed thoracic legs
but lack abdominal prolegs. In other insect larvae, legs
are totally absent; fly and various wasp larvae are
examples.

Classification of insects

Class INSECTA
Subclass APTERYGOTA
Order Thysanura

Order Isoptera Termites

Order Zoraptera Zorapterans

Bristle-tails, silverfish Order Psocoptera Psocids or booklice

Order Diplura Diplurans Order Mallophaga Biting lice

Order Protura Proturans Order Anoplura Sucking lice

Order Collembola Springtails Order Hemiptera True bugs

Subclass PTERYGOTA
Order Ephemeroptera

Order Thysanoptera Thrips

Mayflies Order Neuroptera Alder flies, lacewings, etc.

Order Odonata Dragonflies Order Coleoptera Beetles

Order Plecoptera Stoneflies Order Strepsiptera Stylopids

Order Grylloblattodea Grylloblattodeans Order Mecoptera Scorpion flies

Order Orthoptera Crickets, grasshoppers Order Siphonaptera Fleas

Order Phasmida Stick-insects, leaf-insects Order Diptera True flies

Order Dermaptera Earwigs Order Lepidoptera Butterflies, moths

Order Embioptera Web-spinners Order Trichoptera Caddis flies

Order Dictyoptera Cockroaches, mantids Order Hymenoptera Ants, bees, wasps, etc.