ANATOMY AND PHYSIOLOGY 75 



glands, while the ventral sympathetic system is concerned with the spi- 

 racular muscles. 



5. SENSE ORGANS 



For the reception of sensory impressions from the external world, 

 the armor-like integument of insects is modified in a great variety of 

 ways. Though sense organs of one kind or another may occur on almost 

 any part of an insect, they are most numerous and varied upon the head 

 and its appendages, particularly the antennae. 



Antennal Sensilla. Some idea of the diversity of form in antennal 

 sense organs may be obtained from Figs. 116-125, taken from a paper 

 by Schenk, whose useful classification of antennal sensilla, or sense 

 organs, is here outlined: 



1. Sensillum cceloconicum a conical or peg-like projection immersed 

 in a pit (Figs. 116-117). In all probability olfactory. 



2. S. basiconicum a cone projecting above the general surface (Fig. 

 1 1 8). Probably olfactory. 



3. S. styloconicum a terminal tooth or peg seated upon a more or less 

 conical base (Fig. 119). Olfactory. 



4. S. chaticum a bristle-like sense organ (Fig. 120). Tactile. 



5. S. trichodeum a hair-like sense organ (Figs. 121, 122). Tactile. 



6. S. placodeum a membranous plate, its outer surface continuous 

 with the general integument (Fig. 123). Function doubtful; not audi- 

 tory and probably not olfactory, though the function- is doubtless a 

 mechanical one; Schenk suggests that this organ is affected by air 

 pressure, as when a bee or wasp is moving about in a confined space. 



7. S. ampullaceum a more or less flask-shaped cavity with an axial 

 rod (Figs. 124, 125). Probably auditory. 



These types of sensilla will be referred to in physiological order. 



Touch. The tactile sense is highly developed in insects, and end- 

 organs of touch, unlike those of other senses, are commonly distributed 

 over the entire integument, though the antennae, palpi and cerci are es- 

 pecially sensitive to tactile impressions. 



The end-organs of touch are bristles (sensilla chaetica) or hairs (sensilla 

 trichodea), each arising from a special hypodermis cell and having con- 

 nection with a nerve. Sensilla chaetica doubtless receive impressions 

 from foreign bodies, while sensilla trichodea, being best developed in the 

 swiftest flying insects and least so in the sedentary forms, may be affected 

 by the resistance of the air, when the insect or the air itself is in motion. 



