130 



AN INTRODUCTION TO ENTOMOLOGY 



but the eyes have not been classed among the chemical sense-organs. 

 For these reasons the following groups of sense-organs are recognized: 

 The mechanical sense-organs. — The organs of touch and of hearing. 

 The chemical sense-organs. — The organs of taste and of smell. 

 The organs of sight. — The compound eyes and the ocelH. 

 The cuticular part of the sense-organs. — In most if not all of the 

 sense-organs of insects there exists one or more parts that are of cuti- 

 cular formation. The cuticular parts of the organs of sight and of 

 hearing are described later, in the accounts of these organs; in this 

 place, a few of the modifications of the cuticula found in other sense- 

 organs are described. 



Each of the cuticular formations described here is found either 

 within or at the outer end of a pore in the cuticula ; as some of these 

 formations are obviously setae and others are regarded as modified 

 setae, this pore is usually termed the trichopore; it has also been 

 termed the neuropore, as it is penetrated by a nerve-ending. 



As the cuticular part of this 

 group of sense-organs, those other 

 than the organs of hearing and 

 of sight, is regarded as a seta, 

 more or less modified, these 

 organs are often referred to as 

 the setiferous sense-organs; they 

 are termed the Hautsinnesorgane 

 by German writers. 



Special terms have been 

 applied to the different types of 

 setiferous sense-organs, based on 

 the form of the cuticUlar part of 

 each; but these types cannot 

 be sharply differentiated as 

 intergrades exist between them. 

 In Figure 147 are represented 

 the cuticular parts of several of 

 these different types; these are 

 designated as follows : 



The thick-walled sense-hair, 

 sensillum trichodeum. — In this 

 type the cuticular part is a seta, 

 the base of which is in an alveolus at the end of a trichopore and is 

 connected with the wall of the trichopore by a thin articular mem- 

 brane (Fig. 147, a.) 



Fig. 147. — Various forms of the cuticular 

 portion of the setiferous sense-organs. 

 The lettering is explained in the text. 



