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 antennas 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 
