6 INSECTS. 



number of segments, including, of course, butterflies, beetles, bugs, spiders, 

 scorpions, centipedes, millipedes, not to mention crabs and shrimps, is now, by 

 common consent, used in a much more restricted sense to apply solely to such 

 members of the Arthropoda as have only six walking-legs. In allusion to this 

 feature the class is nowadays often called the Hexapoda, the term being much 

 more precise and applicable than that of Insecta. In addition, however, to the 

 possession of six legs, insects are characterised by certain other well-marked 

 features, serving to distinguish them from all other arthropods. The body is 

 divided into three distinct regions, arranged in a longitudinal series, and named 

 respectively, from before backwards, the head, thorax, and abdomen. 



The head, which varies much in size and shape in different groups, bears the 

 eyes, the antennae, and the jaws. The eyes are of two kinds, simple and compound. 

 The latter, of which there is a single pair, situated one on each side of the head, 

 and often so large as to occupy the greater part of its right and left half, consist 

 externally of a multitude of lenses, often exceeding many thousands in number. 

 The simple eyes, or ocelli, on the other hand, are fewer in number — usually only 

 two or three — and placed upon the forepart of the head. The antennae are movably 

 articulated by means of a special socket to the front of the head, usually below or 

 near the inner edge of the compound eyes. They vary much in structure and 

 length, being sometimes long and pliable, and composed of a large number of 

 segments, as in the cockroach, and at other times short, like those of the house-fly, 

 and consisting of a few 'segments only. There is no doubt that the antennae 

 contain highly important organs of sense, the bristles with which they are studded 

 being probably tactile, and some of the other organs possibly olfactory in function. 



The front edge of the head, or its lower edge when carried vertically, is often 

 movably jointed to the rest of it, and constitutes an upper lip, or labrum. In the 

 formation of the jaws, which are attached to the lower surface of the head, three 

 pairs of appendages, respectively named the mandibles, the maxillae, and the 

 labium, are involved. But these parts are susceptible of an extreme amount of 

 variation in structure and function, being sometimes formed for mastication, as in 

 the mandibulate forms, such as the cockroaches and beetles, and sometimes for 

 piercing or sucking, or both combined, as in the so-called sucking forms like 

 the flies, butterflies, and bugs. There is no doubt that the mandibulate type of 

 mouth in which the gnathites, or jaws, are more foot-like in structure, is the most 

 primitive of all. In this case the mandibles usually consist of a stout pair of one- 

 jointed skeletal pieces, the inner edge of which is furnished with biting teeth. 

 Sometimes, as in the males of stag-beetles, the mandibles are enormously large, 

 and simulate horns. The maxillae are much more complicated in structure ; each 

 consists of a basal piece, composed of two segments — the cardo and stipes — from 

 which spring two branches, an outer or palp, which has the appearance of a 

 dwarfed limb, and an inner, which is in its turn double, the inner blade being called 

 the lacinia, and the outer the galea. The jaws of the third pair, constituting the 

 so called labium, or lower lip, are constructed upon the same principle as the 

 maxillae, but the parts usually considered to correspond to the cardo are united 

 to form a plate — the mentum — which is articulated by its hinder part to a sternal 

 plate of the head, called the submentum. In front of the mentum there are 



