NO. 



MORPPIOLOGY OF INSECT SENSE ORGANS SNODGRASS 



41 



shaped, or peg-like, and whether exposed on the surface or sunken 

 into a pit or deeper cavity of the cuticula, may be grouped together 

 as the hair sense organs, since the cuticular part in each is clearly a 

 modified seta. Under this heading we can distinguish scnsilla trichodea 

 (sense setse), scnsilla chatica (sense bristles), scnsilla squamiforma 

 (sense scales), scnsilla basic onica (sense cones and pegs), scnsilla 

 cadoconica (sense pits), scnsilla anipullacca (sense flasks). 



//-Hr 



.^HrMb 



m 



Fig. 17. — .Sensory hairs of various forms, and a sensory scale. 



A, hair sense organ with distal process of sense cell {d) attached 

 directly to articular membrane (adapted from Hochreuther, 1912, sen- 

 sory hair of Dytiscus) . B, distal process attached by sense rod to base 

 of hair in sense organ of labium of Dytiscus (Hochreuther). C, sense 

 hair with imperforate articular membrane on mandible of Dytiscus 

 (Hochreuther). D, thin-walled sensory hair with terminal strand of 

 sense cell processes attached in tip. E, a solid sensory spine on pharyn- 

 geal plate of Dytiscus (Hochreuther). F, a sensory scale of wing of 

 Notris verbascclla (Freiling, 1909). G. club-shaped sensory hair of 

 cercus of Gryllotalpa vulgaris (Sihler, 1924). 



SENSILLA TRICHODEA 



The sensitized hair is clearly the most primitive of insect sense or- 

 gans, excepting possibly those eyes of certain Dipteran larvae that 

 consist of little else than sensitive hypodermal cells. A sensory hair 

 of the trichodea variety is typically setiforiri, but there is much vari< 

 ation in length and especially in the thickness and density of the hair 

 walls. The longer, stififer sensory hairs are probably all organs of 

 touch ; they are known as tactile hairs. Short hairs with thin transpar- 

 ent walls are usually regarded as organs for receiving stimuli of smell 

 and taste ; they are distinguished as chcmoreceptive hairs. 



Tactile hairs are common in all the major groups of the Arthro- 

 poda ; in insects they occur on most parts of the body and appendages, 



