CHAP. 1. 1 DEFINITION K.ND STRUCTURE OF INSECTS- / 



The eyes oi insects arc of two kinds, simple eyes (called ocelli I 

 and compound eyes, but either compound eyes <>r ocelli maj be 

 totally absent, as in most Termites. The ocelli vary in number 

 from one to twenty or more; when few in number they are 

 generally situated on the top of the head, but when numerous they 

 are usually grouped on either side of the head. They are fre- 

 quently obsolete and often, even when present, are covered with 

 scales and apparently functionless in the adult insect. The 

 compound eyes, which are peculiarly characteristic of insects and 

 are practically only found in adult insects, are composed of 

 numerous separate and self-contained eyes whose exposed suit. 

 form numerous facets of which the whole eye is composed. The 

 compound eyes, each of which may contain thousands of I 

 are usually paired and placed on either side of the head, buttheir 

 shape and position is often much modified in accordance with the 

 habits of the insect concerned. In the predaceous Dragonflies the 

 eyes may occupy practically the whole surface of the head and this 

 is also the case in the males of Tabanid flies, whose females have 

 large but normal eyes. In the Diopsidae the eyes are placed at the 

 extremities of long hornlike stalks which project on either side of 

 the head and it has been suggested that this arrangemenl provides 

 increased accuracy of vision on the principle of a range-finder; 

 these fbes are supposed to be predaceous, but very little is known 

 of their habits. In the Gyrinidas, which are the well-known 

 " Whirligig Beetles " so commonly seen on the surfaces of ponds 

 and streams, the compound eyes are each divided into two portions, 

 so that the beetle is able to see both above and beneath the water 

 whilst it is swimming on the surface. In a curious little fly lately- 

 described from South Africa the compound eyes are three in num- 

 ber, one on each side and one on top of the head ; the object of this 

 triple distribution is not apparent. It may be added that the lenses 

 of the compound eyes are often unequal in size and shape in 

 different portions of the eye and that this fact is sometimes useful 

 for the discrimination of species. 



The appendages of the Thorax. 

 The legs are affixed in pairs to the lower surfaces ol the three 

 thoracic segments and are hence six in number. They are almost 

 always present both in the larval and perfect stages, although 

 there are many totally legless larvae, especially amongst the 

 Diptera and Hymenoptera, whilst in the adult insect some of the 

 legs maj be specially modified, obsolescent or obsolete- In the 



• The number "I fai I is ven variabli : in some ants there 



ure ..nh about 50. in dragonflies there ma> be as many as 12,000 and in ■."inc beetles 

 (Morticlla) more than 25,000. 



