THE INTERNAL ANATOMY OF INSECTS 131 



If the sense-hair is short and stout, it is termed by some writers 

 a sense-bristle, sensillum chceticum; but there is Httle use for this dis- 

 tinction. 



In the thick-walled sense-hairs, the wall of the seta is fitted to 

 receive only mechanical stimuli, being relatively thick, and as these 

 organs lack the characteristic features of the organs of hearing, they 

 are believed to be organs of touch. 



The sense-cones, — The sense-cones vary greatly in form and in their 

 relation to the cuticula of the body-wall ; their distinctive feature is 

 that they are thin-walled. For this reason, they are believed to be 

 chemical sense-organs, the thinness of the wall of the cone permitting 

 osmosis to take place through it. In the sense-cones, too, there is no 

 joint at the base, as in the sense-hairs, the articular membrane being 

 of the same thickness as the wall of the cone ; there is, therefore, no 

 provision for movement in response to mechanical stimuli. 



In one type of sense-cone, the sensillum basiconicum, the base of 

 the cone is at the surface of the body- wall (Fig. 147, 6). In another 

 type, sensillum cceloconicum, the cone is in a pit in the cuticula of the 

 body- wall (Fig. 147, c). Two forms of this type are represented in 

 the figure ; in one, the sense-cone is conical ; in the otha", it is fungi- 

 form. Intergrades between the basiconicimi and the coeloconicum 

 types exist (Fig. 147, d). 



The flask-like sense-organ, sensillum ampullaceum. — This is a 

 modification of the sense-cone type, the characteristic feature of 

 which is that the cone is at the bottom of an invagination of the articu- 

 lar membrane; in some cases the invagination is very deep so that 

 the cone is far within the body- wall (Fig. 147, ^) ; intergrades between 

 this form and the more common sensillum coeloconicum exist (Fig. 



147. /)• 



The pore-plate, sensillum placodeum. — In this type the cuticular 

 part of the organ is a plate closing the opening of the trichopore ; in 

 some cases, this plate is of considerable thickness with a thin articular 

 membrane (Fig. 147, g); in others it is thin throughout (Fig. 147, /i). 



The olfactory pores. — This type of sense-organ is described later. 



X. THE ORGANS OF TOUCH 



The organs of touch are the simplest of the organs of special sense 

 of insects. They are widely distributed over the surface of the body 

 and of its appendages. Each consists of a seta, with all the character- 

 istics of setae already aescribed, a trichogen cell, which excreted the 



