THE NERVOUS SYSTEM 121 



All the mechanical sense organs, besides the specific effects which 

 stimulation of them elicits, have the property of arousing the nervous 

 system generally, and bringing it into a reactive condition. They be- 

 long to the so-called 'stimulatory organs' which serve both to main- 

 tain the muscle tone, and to enhance the kinetic efficiency of the 

 muscles when they are brought into action. Prominent among the 

 stimulatory organs are the halteres of Diptera and Strepsiptera, one 

 important function of which is to sustain the muscular vigour of the 

 insect during flight by the stimuli which their own vibration gener- 

 ates. In this case the receptors seem to be partly the chordotonal 

 organs but chiefly the abundant campaniform sensilla. 



Chemical senses 



The chemical senses of taste and smell are developed in varying 

 degree. Accepting the definition that what we perceive by the tongue 

 are tastes and the flavours we detect by the nose are odours, insects 

 can recognize the tastes of salt, sweet, acid, and bitter by means of 

 receptors in the mouth (Apis, &c), on the antennae (Hymenoptera), 

 on the palpi (Dytiscus, Geotrupes, &c.) and on the tarsi (Diptera, 

 Lepidoptera) - often in several of these sites in the same insect. 

 They can detect odours chiefly by sensilla on the antennae, but to a 

 lesser degree by the palps. While the 'general chemical sense', affected 

 by pungent vapours, is present in many parts of the body. Receptors 

 for the perception of specific flavours occur in the pharynx of sucking 

 insects, notably the Hemiptera; thus the blood-sucking bug Rhodnius 

 will take up salt solution through a membrane more readily if a 

 trace of haemoglobin is added to it. 



The chemoreceptive sensilla are usually thin-walled hairs or pegs. 

 The distal nerve processes from the sense cells break up inside the 

 cavity of the hair to produce a great number of fine filaments, visible 

 only with the electron microscope, which penetrate the thin walls of 

 the sensillum and come to have their extremities freely exposed to the 

 atmosphere. In some of the larger sensory hairs, such as those on the 

 tarsi or labella of blow flies, there are three or four sense cells giving 

 off separate distal processes which serve different purposes - to 

 perceive sugars, salts, repellant substances, or water. 



The sensitivity of the organs of taste in insects may be very high; 



