Glass 3. Insecta. 



231 



Class 3. Insecta. 



The insectan body is divided into three sections : head, thorax, 

 and abdomen. The head is sharply marked off from the thorax, 

 and is usually freely movable : on each side there is a sessile com- 

 pound eye consisting of a very large number (twenty to many 

 thousands) of small ocelli, covered externally by convex facets ; 

 each of these is usually hexagonal in shape, and corresponds in 

 position with an ocellus. In many Insects the eyes occupy a very 

 large part of the head (in many Diptera, for instance, almost the 

 whole of it). In form they are most frequently circular, but often 

 kidney-shaped, and so on. Occasionally the compound eyes are 



Pig. 194. Antennae of various Insects. 1 bristle-like, 2 filiform, 3 moniliform, 

 4 pectinate, 5 — 7 clavate (7 with laminate club). — After Judeich and Nitsche. 



replaced by small groups of ocelli* (Collembola), or by a single 

 ocellus on each side (Pleas and Lice). In many Insects, there 

 are from one to three ocelli on the middle of the head, ia addition 

 to the compound eyest (c/., the nauplius eye of Crustacea) . Prom the 

 head, arises a pair of antennae or feelers, which either 

 consist of a limited number of well-developed joints, or of a large 

 number of very short ones. The form of the antennae is very varied ; 

 at the simplest, they are filiform or bristle-like, but they are some- 

 times moniliform (much, constricted at the joints), pectinate (the 

 joints being produced on one or on both sides into processes), or 

 clavate (club-shaped). 



The head also bears the mouth and the surrounding mouth- 

 parts, which vary greatly in form, though all may be referred 

 to a common type. The simplest and most primitive condition is 

 presented by the biting mouth-parts of the Orthoptera, the 

 Coleoptera, the Neuroptera, and the Hymenoptera, where an upper 



* Only the adult Insect is referred to here ; for larval arrangements, see below, 

 f Absent from nearly all Beetles. 



