8 AN INTRODUCTION TO ENTOMOLOGY, 



The sclerites constitute the greater part of the body-wall, the soft 

 membranous portions separating them being in most cases narrow. 



Usually these narrow portions are mere 

 lines ; they are then called sutures. 



Frequently the sutures become en- 

 tirely effaced. We are therefore often 

 unable to distinguish certain sclerites 

 in one species of insect which we know 

 to exist in another. In such cases the 

 effaced sutures are said to be obsolete. 

 Fig. i 5 .— Poiistes beiikosa. The segments of the body in a fully 



developed insect are grouped into three regions : head, thorax, and 

 abdomen (Fig. 15). In the larval state this grouping of the segments 

 is not well shown. 



The Head. 



The head is the first of the three regions of the body. It is sup- 

 posed to be formed of several body-segments grown together; but 

 entomologists differ in their views as to the number of segments 

 that have entered into its composition. 



It does not fall within the scope of this work to enter into this discussion. 

 The main point, however, can be stated here. A careful study of the various 

 forms of Arthropods shows that the typical body-segment possesses a pair of 

 legs, and only one pair. It is known that certain mouth parts (mandibles, 

 maxillce, and labium) are modified legs. (This fact is easily seen in many 

 Crustacea.; The antennae and the eyes may also be modified legs.* It follows 

 that if we find represented in the appendages of the head the appendages of 

 several segments, the head itself must consist of several segments coalesced. 



The principal portion of the chitinized parts of the head are firm- 

 ly joined together so as to constitute a box which contains the brain 

 of the insect and certain other important organs. To this are artic- 

 ulated a number of jointed appendages. The parts of the head 

 may be classed, therefore, under two divisions : first, the fixed parts ; 

 second, the movable parts. 



The Fixed Parts of the Head. 



In addition to the external portions of the organs of vision (the 

 compound eyes, and the simple eyes), the fixed parts of the head 

 consist of four sclerites. Three of these sclerites (occiput, epicra- 



* The belief that the eyes are modified legs is based on the fact that in certain Crusta- 

 cea the eyes are situated on stalks which are jointed appendages of the head. 



