These are the Diplura (bristletails), Collembola (springtails), Protura and Thysanura (silverfish).
The bristletails and silverfish both have two long cerci, and between this a caudal appendage. There are just under 10,000 described species of Apterygotes.
These insects have two pairs of wings; one pair on the mesothorax and another on the metathorax. Pterygotes have muscles which allow them to close their spiracles so greatly reducing water loss and enabling them to colonize drier habitats.
The Pterygotes can be further divided into two groups; those that undergo complete metamorphosis (holometabolous), and those that undergo gradual metamorphosis (hemimetabolous).
Hemimetabolous insects have young that resemble the adults, and the gradual development of wings is external . Such insects include, Cimex lectularius (the bed bug) above, which shows the gradual change to the adult form, also Odonata (dragonflies), Orthoptera (grasshoppers), Dermaptera (earwigs) and Hemiptera (aphids, cicadas).
A typical holometabolous insect the brown house moth, Hofmannophila pseudospretella shown on the below in the various stages of its life.
Holometabolous insects have young (larva) that usually do not resemble the adult (see above), and often occupy a different niche. These insects form a pupa (see above) before emerging as the adult form.
About 88% of the described insects are holometabolous and include the four largest Orders that have diversified along with the diversification and radiation of the Angiosperms; they are the Diptera (flies), Coleoptera (beetles), Hymenoptera (wasps, ants, bees), and Lepidoptera (butterflies and moths).
| Disease | Insect vector |
| Malaria | Anopheles mosquito (Diptera) |
| Yellow fever | Aedes aegypti Mosquito (Diptera) |
| Filariasis | Culex mosquito (Diptera) |
| Typhoid | House fly (Diptera) |
| Dengue | Aedes mosquito (Diptera) |
| Sleeping sickness | Tsetse fly (Diptera) |
| Onchocerciasis (river blindness) | Simuliidae |
| Leishmaniasis (sand fly fever) | Psychodidae |
| Plague | Fleas (Siphonaptera) |
| Typhus fever | Louse (Anoplura/Siphunculata) |
| Chagas | Rhodnius (Hemiptera) |
Courtship. This usually involves males trying to attract females. The female behaviour is simply to accept or reject, e.g. during courtship the male scorpion fly presents the female with a dead insect which she eats during copulation.
Copulation. Copulation is the physical joining of the male and female culminating in the passing of sperm from the male to the female, e.g. bumblebees.
Egg-laying. Most species have very specific site requirements for egg-laying, so that the eggs are laid on, in or near the source of food for the larva or nymph, e.g. dung beetles.
Defence. Defence can be active, e.g. sting (bees, wasps, etc.), bite (beetles, etc.), chemicals (bombardier beetle); or passive e.g. hiding (cockroach), camouflage (stick insect).
Communication. Communication is the transfer of information between individuals - usually of the same species, e.g. honeybees communicating the location of a good food source.
Grooming. This is especially important in hairy insects, e.g. bumblebees. These and all others need to keep their antennae, eyes, wings, legs and mouthparts clean. Most have a brush or comb on their forelegs, e.g. bumblebees, that can be used to cleans antennae and legs.