SURVEY OF INVERTEBRATES 



93 



A typical Insect is characterized by a body divided into three 

 major parts: head, thorax, and abdomen. The head bears a pair 

 of compound eyes, usually one to three simple eyes (ocelli), a 

 pair of 'feelers' (antennae), and the mouth parts: the labrum, 



MANDIBLES, MAXILLAE, and LABIUM. (FigS. 54, 116, 215.) 



The thorax is composed of three segments, each of which typi- 

 cally bears a pair of legs. The legs of Insects in most cases per- 

 form many functions in addition to locomotion: they are really 

 a set of tools. Witness the legs of the Honey Bee. Usually two 

 of the three thoracic segments each bear a pair of wings, but the 

 House Fly, of course, has but one pair and the Flea none at all, 

 though the Fly's 'balancers' are remnants of its missing pair. The 

 wings of Insects are entirely dissimilar in origin and structure 

 from the legs and therefore bear no relation to the paired append- 



Head Thorax 



Abdomen 



A 



Antennae 



Auditory organ 



Femur ^ Spiracles 



Tibia 

 Tarsus 

 Fig. 54A. — Grasshopper, or Locust, Melanoplus differentialis. 



ages of other Arthropods. They are new structures that confer upon 

 them the honor of being the only Invertebrates to conquer the 

 air. Indeed, their adaptive radiation to all sorts of habitats and 

 modes of life exhibits a versatility that somewhat parallels that 

 of the highest Vertebrates, the Mammals. (Figs. 53, 208, 257, 

 258, 261.) 



And moreover, certain Insects excel all the rest of the whole 

 living world, except Man, in the remarkable development of com- 

 munal organization. This involves specialization of individuals 

 for definite contributions to the economy of the social unit, such 

 as the Ant nest or the Bee hive. (Figs. 214-219, 222.) 



