THE INTERNAL ANATOMY OF INSECTS 



133 



organs are widely distributed over the body and its appendages, and 

 in some parts, as on the antennae of many insects, several different 

 types of sense-organs are closely associated. 



Those organs that are characterized by the presence of a thin- 

 walled sense-cone (Fig. 147, b-f) or by a pore-plate (Fig. 147, g, h) are 

 believed to be chemical sense-organs. It is maintained by Berlese 

 ('09, a) that an essential feature of these chemical sense-organs is the 

 presence of a gland -cell, the excretion of which, passing through the thin 

 wall of the cuticular part, keeps the outer surface of this part, the 

 sense-cone or pore-plate, moist and thus fitted for the reception of 

 chemical stimuli. According to this view a chemical sense-organ 

 consists of a cuticular part, a trichogen cell or cells which produced 



3i 



Fig. 148. Sections through the body-wall and sense-hairs of the silk- 

 worm; c, cuticula; h, hair; hy, hypodermis; n, nerve; 5, bipolar 

 nerve-cell (From Hilton). The line at the right of the figure indi- 

 cates one tenth millimeter. 



this part, a gland-cell which excretes a fluid which keeps the part 

 moist, and a nerve-ending. 



It is interesting to note that tactile hairs may be regarded as 

 specialized clothing hairs, specialized by the addition of a nervous 

 connection, and that sense-cones and pore-plates may be regarded as 

 specialized glandular hairs with a nervous connection; in the latter 

 case, the specialization involves a thinning of the wall of the hair so as 

 to permit of osmosis through it. 



In the different accounts of chemical sense-organs there are 

 marked differences as regards the form of the nerve-endings. In 

 many of the descriptions and figures of these organs the nerve-ending 

 is represented as extending unbranched to the chitinous part of the 

 organ, resembling in this respect those represented in Figure 148. 

 In other accounts the gland-cell is surrounded by an involucre of 

 nerve-cells (Fig. 149). 



