13-t 



AN INTRODUCTION TO ENTOMOLOGY 



In the types of chemical sense-organs described above the action 

 of the chemical stimuli is supposed to be dependent upon osmosis 



through a delicate 

 cuticular membrane. 

 It should be noted, 

 however, that sev- 

 eral writers have de- 

 scribed sense-cones in 

 which there is a pore ; 

 but the accuracy of 

 these observations is 

 doubted by other 

 writers. 



A very different 

 type of sense-organs 

 which has been termed 

 olfactory pores is de- 

 scribed in the conclud- 

 ing section of this 

 chapter. 



Fig. 149.— Section of the external layers of the wall of 

 an antenna of A crida turrita; Ct, cuticula; Ip, hypo- 

 dermis; N, nerve; Nv, involucre of nerve-cells sur- 

 rounding the glandular part of a sense-organ; Sbc, 

 sensillum basiconicum; Sec, sensillum coeloconicum. 

 Three sense-organs are figured; a surface view of the 

 first is represented, the other two are shown in sec- 

 tion. (From Berlese). 



XII. THE ORGANS OF SIGHT 



a. THE GENERAL FEATURES 



The two types of eyes of insects. — It is shown in the preceding 

 chapter that insects possess two types of eyes, the ocelli or simple eyes 

 and the compound or facetted eyes. 



Typically both types of eyes are present in the same insect, but 

 either may be wanting. Thus many adult insects lack ocelli, while 

 the larvae of insects with a complete metamorphosis lack compound 

 eyes. 



When all are present there are two compound eyes and, typically 

 two pairs of ocelli ; but almost invariably the members of one pair of 

 ocelli are united and form a single median ocellus. The median ocel- 

 lus is wanting in many insects that possess the other two ocelli. 



The distinction between ocelli and compound eyes. — The most 

 obvious distinction between ocelli and compound eyes is the fact that 

 in an ocellus there is a single cornea while in a compound eye there are 

 many. Other features of compound eyes have been regarded as dis- 

 tinctively characteristic of them; but in the case of each of these 

 features it is found that they exist in some ocelli. 



