ANATOMY OF INSECTS - - EXTERNAL 



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further specialized as organs of locomotion. With the development 

 of the large muscles necessary for the propulsion of the wings and 

 legs, these three segments back of the head became sharply differ- 

 entiated from the rest, so that they now form a quite distinct 

 region, the thorax. The remaining posterior segments, called the 

 abdomen, having lost most of their appendages, are quite similar 

 in form, with the exception of those at the extreme posterior end, 

 where the shape of the segments and of their appendages has 

 been modified in connection with the external sexual organs. The 

 insect is therefore divided into three well-defined parts,- -the head, 

 the thorax, and the abdomen, - - which are composed of more or 

 less visible segments. 



The head. The embryology 

 and nervous system of the head 

 show that it was originally com- 

 posed of six segments, almost 

 no traces of which are now 

 discernible except their append- 

 ages, of which four pairs are rec- 

 ognizable as homologous with 

 the thoracic legs and the ab- 

 dominal appendages of lower 

 forms. These appendages con- 

 sist of the feelers, or antennae, 

 and three pairs of mouth-parts. 

 The head also bears a pair of 

 compound eyes and often a variable number of simple eyes, or ocelli. 



Antennae. The antennae are often called feelers, indicating their 

 principal function as sense organs, which will be discussed in con- 

 sidering the senses. The shape of the antennae is very different 

 in different groups of insects, as is also the number of segments, 

 both of which characters are of the greatest importance in dis- 

 tinguishing the various groups. In the case of the katydid the 

 threadlike antennae are much longer than the body, while in some 

 flies they are reduced to mere knobs with a single strong bristle. 

 The different shapes of the segments give rise to many different 

 characteristic types of antennae, some of the more important of 

 which are shown in Fig. 8. In many cases, notably in the moths 



FIG. 9. Head of drone bee, showing 

 compound and simple eyes 



(After A. B. Comstock) 



