INTERNAL ANATOMY. 113 



Tyroglyphida3 are very few, and little is known about 

 them except that they are extremely simple. Probably 

 the great leading sense in all Acari is touch ; this no 

 doubt is widely distributed over the body ; but, as far 

 as can be judged, it is located principally in certain 

 long setiform hairs one of which is placed on the 

 median point of the dorsal (or anterior) side of each 

 tibia of the first pair of legs, near the distal end of the 

 joint. A similar hair, similarly placed, but usually not 

 quite so long as that on the first leg, is found on each 

 tibia of the second pair of legs ; and very often on each 

 tibia of the third and fourth pairs ; but these hairs are 

 usually shorter than those of the second pair. In 

 treating of other families of Acari I have always called 

 these particular hairs "the tactile hairs," not as in- 

 tending to indicate that they are the only hairs on the 

 creature which have a tactile sense ; but as a name 

 which drew attention to the fact that it is in them that 

 the highest tactile sense is resident. These hairs are 

 found practically in all non-parasitic Acari, and in many 

 parasitic ones ; but the extent to which they are de- 

 veloped in different families and species varies con- 

 siderably. The sense of touch, and probably the 

 sensitiveness to minute disturbances of the air which 

 these hairs possess, must be very high ; for it is appa- 

 rently by their means that the creatures avoid col- 

 lisions, etc., when they are in rapid movement ; and it 

 is apparently by their assistance that such blind, preda- 

 tory Acari as Gamasus are enabled to catch and devour 

 such active insects as the Thysanuridge. It must be 

 remembered that in consequence of the perpendicular 

 position of the tibise during life these hairs stand out 

 horizontally from the tibia and point forward. 



The sense of touch also probably exists in a lesser 

 degree in many of the other hairs upon the creature ; 

 there is also every reason for believing that it is rather 

 efficient in the palpi. In such highly-developed 

 families as the GramasidaB these organs are constantly 

 trembling, and touch the ground lightly and rapidly at 

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