CHAPTER III 



Mechanoreception 



Every insect maintains its body in a particular attitude with respect to 

 gravity. This attitude, its primary orientation, means for most 

 ventral-side down. There are, however, many that habitually live 

 upside down (the backswimmers, praying mantids, many lepi- 

 dopterous larvae) and a few, such as those living in vertical burrows, 

 whose normal attitude is posterior-end down. For aquatic insects 

 and flying insects, both of whom are freely suspended in a homo- 

 geneous medium, there must be sense organs that can give information 

 about the direction of the force of gravity. Vision is of some help in 

 this connexion, especially in flying insects, since the sky is invariably 

 up and the ground down, but that is not the prime service of vision. 

 Insects in contact with a substrate derive some information from 

 sense organs touching the substrate, but this information tells them 

 only which surface of the body is in contact, not what the attitude of 

 their body is with respect to gravity. 



In addition to maintaining a primary orientation all insects have 

 characteristic postural relations. To maintain these, to have infor- 

 mation concerning the position of one part of the body with respect to 

 another, requires special sense organs. The same sort of information is 

 needed to be able to move one part of the articulated body precisely 

 with respect to another in order to walk, swim, fly, spin cocoons, dig 

 burrows, court, copulate, and feed. Finally, for the proper working of 

 the internal organs there must be information concerning the presence 

 of faeces in the rectum, eggs in the ootheca, and food in the alimentary 

 canal. 



Serving all of these needs is a whole spectrum of sense organs 

 responding to energy derived, in the final analysis, from the surface 

 gravitation of the earth. They are organs sensitive to stretch, com- 

 pression, or torque imparted to cuticle, connective tissue, or muscles 

 by the weight (which is a measure of the force with which the earth 

 attracts it) of parts of the body, the relative movement of parts, the 

 gyroscopic effects of moving parts, and impingement of the substrate 

 or surrounding media. Since there is no neuron directly sensitive to 

 gravitational force, all of the receptors concerned with primary 



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