INSECTS 345 



Among Insects the Cockroach is a fairly central type, and 

 the least modified part of it is the abdomen, in which ten seg- 

 ments can clearly be distinguished. A pair of flattened jointed 

 rods, the cerci, spring from the last segment, and are probably 

 to be regarded as limbs, which are otherwise absent in this region, 

 if certain doubtful structures be excepted. This practically 

 limbless condition of the abdomen is characteristic of insects, and 

 has been brought about by reduction. The thorax is commonly 

 regarded as made up of three segments, and each of these bears 

 a pair of jointed limbs or appendages in the shape of walking- 

 legs. Each leg is composed of several pieces (lo) movably jointed 

 together, a character which is common to all Arthropod limbs, and 

 has given the name to the phylum (Gk. arthros, jointed; pons, 

 a foot). The parts of the leg differ much among themselves 

 in respect of size and shape. The thorax also bears a pair of 

 flattened expansions, forming the wings, attached to its second 

 and third segments, which in the male of the common Cock- 

 roach, and both sexes in the American species, extend far back- 

 wards and overlap the abdomen. The fore-wings are horny 

 structures, and may be termed the wing-covers, since they cover 

 and protect the delicate membranous hind-wings, or wings proper. 

 In the female of the ordinary Cockroach the fore-wings are very 

 small and the hind-wings absent, reduction having taken place in 

 both cases. 



The head consists of a number of segments which have fused 

 so intimately together that the boundaries between them cannot 

 be made out. As, however, in the higher Arthropods the presence 

 of pairs of jointed limbs is taken to indicate the existence of a 

 corresponding number of segments, a certain clue is afforded, 

 though, as all segments do not bear limbs, such evidence is 

 only partial. In this case, as in Insects generally, there are 

 four pairs of appendages, so that the head possesses at least 

 four segments. The first of these are two slender jointed feelers 

 or antenna, serving as organs of touch, and probably also having 

 to do with other senses. The remaining appendages are three 

 pairs of jaws, named, from before backwards, mandibles, first 

 maxillae, and second maxillae. As in all Invertebrates they are 

 quite outside the opening of the mouth, and those of them which 

 are used for biting work from side to side. Each mandible con- 

 sists of a single broad horny piece, toothed on its inner edge. 



