HEXAPODS. 131 



Sub-Class IV. — Hexapoda. 



The Hexapoda, or insects proper, derive their systematic name, which means six- 

 footed, from the fact that the adults (with a few inconspicuous exceptions) have three 

 pairs of feet fitted for locomotion. Besides this there are many other correlated and 

 distinctive characters, and the group is as well defined and as closely circumscribed as 

 any in the animal kingdom. The body of all hexapods is divided into three well- 

 mai-ked regions, — head, thorax, and abdomen, — each composed of a number of 

 segments. In the larval stages these distinctions are frequently not so easily traced 

 as in the adult, but still they can be readily recognized. In our discussion of the 

 morphology of the hexapods we will first give the characters of the adult and after- 

 wards those of the larval and pupal stages. 



The head, which, as has been stated on a preceding page, consists of four segments, 

 bears a pair of compound eyes and usually two or three simple eyes in addition. There 

 is but one pair of feelers or antennae, and three pairs of mouth-parts. The most 

 anterior of the appendages connected with the mouth are the mandibles, which are 

 always without the palpi so common in the Crustacea ; next come the maxillae, and 

 then the labium, both of which bear articulated prolongations known respectively as 

 maxillary and labial palpi. Besides these, there are other parts concerned in eating, 

 which, though not appendages, deserve mention here. These are the labrum or upper 

 Up, and the metastoma, lingula or tongue. These mouth-parts ai-e variously modified in 

 different insects, thus affording most important systematic characters. In the Lepi- 

 doptera they form a long sucking-tube which, when not in use, may be coiled in a spiral, 

 while in the Hemiptera and many Diptera, in addition to being adapted for suction, 

 they form a piercing organ. In all the other orders the mouth-parts retain more 

 nearly their primitive character, and are fitted for biting. 



The second region of the body, the thorax, is composed of three segments, each of 

 which bears a pair of legs, and in addition the two last usually bear a pair of wings. 

 To each of these rings of the thorax a distinctive name is applied, and since these 

 portions are very important and often referred to the names may here be given. The 

 first is the prothorax, the second mesothorax, and the third and last metathorax. 

 Sometimes all of these rings are distinct and equal, but more frequently one is en- 

 larged at the expense of the other two. Thus in the Hemiptera, Coleoptera, and 

 many Orthoptera, the prothorax is very large, while in the others it is relatively 

 much smaller. 



Of the legs we need here say nothing, referring the reader to page 90 of this 

 volume. The wings, however, deserve a passing notice. According to the older 

 views of morphology these organs were regarded as modified limbs, homologous with 

 the legs, but the present view regards them as but extensively-developed folds of the 

 skin, comparable to the respiratory organs. The wings never exceed two pairs in 

 number, and in the Diptera only one pair is well developed, the posterior being 

 represented by little stalked balls, the halteres. Usually the wings are strengthened 

 by veins, each of which has its proper name. Though the patterns of venation in 

 many forms seem very complex, they arc readily referable to a common plan, the 

 outlines of which may be obtained in any handbook of entomology. 



