1. Lobster (Astacus marinus). 2. Cray-fish (Astacus fluviatilis). 3. Crab (Cancer pagurus). 4. Shrimp (Crangou vulgaris). 5. Prawn (Palæmon serratus). ] [Illustration: FIG. 16.--INSECTS. 1. Dragon-fly (Libellulina depressa). 2. Grasshopper (Gryllus). 3. Bee (Apis mellifica). 4. Fly (Musca domestica). 5. Butterfly (Pontia brassicæ). 6. Musk-beetle (Cerambyx moschatus). ] The Insects form the fourth class of this division, and by far the most important and numerous. They come to maturity only after undergoing successive changes from the egg to the perfect animal; these transformations are amongst the most wonderful things connected with them. The eggs (some of which are very beautiful) are first deposited in some safe place, either attached to a leaf or tied up in a small bundle by silken threads spun by the parent insect, and in some nutritious substance, so that when it comes to life it may at once have food; this is sometimes in manure, sometimes in flesh, and sometimes under the skin of a living animal (few are exempt from this infliction), where they remain for a time and then come forth as maggots, caterpillars, &c.; in this state they are called "larvæ," these are generally active creatures and eat most voraciously, which seems to be the principal act of this state of their existence. These larvæ frequently change their skins as they grow, and at last they assume the next stage of their life, the pupa or chrysalis state, which is one generally of complete inactivity; many of these larvæ, weave themselves a covering of a sort of silk, to defend them while in the pupa state--such as the Silkworm, whose covering (cocoon) is the source of all the silk of commerce--others merely place themselves in a situation of security. The pupa remains dormant for a certain time, and then becomes the imago or perfect insect (the last state of its existence), such as a Moth, a Butterfly, a Beetle, &c. These are of different sexes, and in due time produce a batch of eggs and then die; these eggs are often incredible in numbers, amounting to many thousands--fortunately but few escape the watchful eyes of other insects and of birds who feed upon them. [Illustration: FIG. 17.--ANTENNA OF COCK-CHAFFER (Melolontha vulgaris).] [Illustration: FIG. 18.--FOOT OF FLY, MAGNIFIED.]