30 AN INTRODUCTION TO ENTOMOLOGY. 



mentary, and doubtless the sense of smell likewise. During the 

 night these insects are passive, while during the day they trust to 

 their powers of sight, or possibly in some Cicadas also to hearing. 



The study of the sensations of insects is at the same time one of 

 the most difficult and one of the most interesting of the departments 

 of entomology. But as yet comparatively little has 

 been done in this line. The great improvements that 

 have recently been made in the methods of histologic- 

 al research have resulted in the publication of several 

 very important papers on the structure of sense-organs ; 

 but there is a great lack of experimental basis for con- 

 clusions as to the functions of the various organs that 

 have been described.* 



The best known of the organs of special sense are 

 the organs of sight. Of these there are two kinds, 

 the simple eyes and the compound eyes. Simple 

 eyes exist in both larvae and adult insects. In the 

 former there may be several of these, on each side of 

 the head ; in the latter there are usually not more 

 than three, situated between the compound eyes. 

 The compound eyes occur only in adult insects, where 

 they reach a marvellous degree of complexity. Each 

 compound eye consists of many ocelli united ; the 

 number varies from 50 in some ants to more than 

 30,000 in certain butterflies. The complexity of these 

 FIG. 45. Three eyes does not, however, consist merely of the great 



ocelli with retinu- , r n . .-, . ., . . r 



la from the com- number of ocelli that enter into the composition of 

 May-bee e tTI.(Afte^ each ; but each ocellus is a highly developed organ 



Grenacher.) The . . , ~, r f 



pigment has been consistingot many parts. 1 he structure or these ocelli 



dissolved away . . . .. rf . >. 



from two of them, vanes greatly in aiiierent insects. t risf. 4^ repre- 



f, corneal facet; * . 



K, crystalline sents three ocelli of a May-beetle as described by 



cone; /, pigment- 



sheath; p, chief Grenacher, 



pigment-cell; /", . 



pigment -cells of With regard to the organs of special sense other 



the second order; ** 



R, retinuiae. than those of sight there is much doubt. It is prob- 

 able that many insects possess organs of hearing ; for elaborate 

 sound-producing organs are common among them. But the only 



* For a general discussion of this subject, and for many references to the literature, 

 see Experiences et Remarques critique sur les Sensations des Inseftes, par Augttste For el, 

 Recueil Zoologique Suisse, t. IV. (1886). 



f See B. T. Lowne, On the Modifications of the Simple and Compound Eyes of In- 

 sects (Philos. Trans. 1878); also the works cited by this author. 



