INSECTS 
With the Fishes w^e concluded the survey of the back- 
boned animals or Vertebrates, one of the main groups of the 
animal kingdom. These main groups are termed ' sub-king- 
doms ' or * phyla.' The Insects which are the subject of the 
present chapter, constitute the highest class of another sub- 
kingdom or phylum, that of the Arthropoda (i.e. animals with 
jointed legs), this phylum including also Centipedes and 
Millipedes, Spiders and Scorpions, and Crustaceans. Insects 
invariably have six legs and have thus appropriately also been 
called Hexapoda. The name ' Insects ' refers to the division 
of the body into three sections, head, thorax and abdomen. 
Most Insects also possess two, or more rarely one pair of wings, 
distinguishing them from all other Arthropods, and. in common 
with the Centipedes, Millipedes, Spiders and Scorpions, they 
possess air tubes or tracheae which permeate the body and 
serve as respiratory organs, whilst the Crustaceans have gilts. 
The Arthropods breathing by means of those air-tubes have 
therefore been called Tracheata. The body of Insects is en- 
closed by a horny or chitinous covering, and the head is provi- 
ded with one pair of feelers or antennae. 
Everybody knows that from the eggs of Butterflies 
not Butterflies are hatched, but caterpillars, that these cater- 
pillars, after having spent their whole existence in feeding and 
having thus greatly increased in size, change into a pupa or 
chrysalis, and that after a shorter or longer quiescent stage the 
perfect insect (or ' imago *J emerges from the chrysalis. It is 
similar with beetles and flies, only there the larva is not called 
caterpillar, but grub and maggot respectively. In Cockroaches 
and Grasshoppers on the other hand there is no such metamor- 
phosis. The young ones emerging from the eggs resemble more 
or less their parents, except that they are much smaller 
and have no wings. In other Insects again we find an inter- 
mediate state of affairs, The character of this metamorphosis 
together with the number and structure of the wings and the 
structure of the mouth parts, according whether they are biting 
