84 ENTOMOLOGY 



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 appen- 

 dages, particularly the antennae. 



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

 sense organs may be obtained from Figs. 118-127, taken from a paper 

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

 organs, is here outlined: 



i. Sensillum codoconicum a conical or peg-like projection immersed 

 in a pit (Figs. 118, 119). In all probability olfactory. 



2.5. basiconicum a cone projecting above the general surface (Fig. 

 120). Probably olfactory. 



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

 less conical base (Fig. 121). Olfactory. 



4. S. chceticum a bristle-like sense organ (Fig. 122). Tactile. 



5. S. trichodeum a hair-like sense organ (Figs. 123, 124). Tactile. 



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

 with the general integument (Fig. 125). Function doubtful; not audi- 

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

 mechanical one; Schenk su'ggests 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. 126, 127). 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 

 especially 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 connection 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. 



Not all the hairs of an insect are sensory, however, for many of them 

 have no nerve connections. 



In blind cave insects the antennae are very long and are exquisitely 

 sensitive to tactile impressions. 



Taste. The gustatory sense is unquestionably present in insects, as 

 is shown both by common observation and by precise experimentation. 



