2968 THE JOINTED ANIMALS 



limbs, and in having no antennae, strikingly resemble the Arachnida. Peripatus is 

 very peculiar, but shows signs of distant relationship with the centipedes, although 

 in many anatomical features it is not very far removed from the worms. 



The term insect, although originally and, according to the meaning 

 of the word, correctly employed in a wide sense to embrace all those 

 teristics animals in which the body is externally divided into a number of seg- 

 of Insects ments, including, of course, butterflies, beetles, bugs, spiders, scor- 

 pions, 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 pos- 

 session of six legs, insects are characterized 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, consists 

 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 fore part of the head. The antennae are mova- 

 bly 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 seg- 

 ments, 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 con- 

 tain 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 mandi- 

 bulate forms, such as the cockroaches and beetles, and sometimes for piercing or suck- 

 ing, 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 gnat- 

 hites, 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', 1 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 



