I20 The Method of Ovipositing. 



twenty to fifty. We must not ascribe any mechanical 

 function in the process of egg-laying to these delicate 

 hairs ; they are merely tactile organs, each hair being 

 in connexion with a nerve fibre. These nerve fibres 

 all arise from the large abdominal ganglion, which also 

 gives off motor fibres to the piercing apparatus \ The 

 tactile hairs scattered over the arch have the important 

 function of informing the fly as to the true position of 

 the ^%^^. While the ^^'g is gliding over the hard 

 chitinous covering of the ovipositor, the fly only becomes 

 conscious of its progress from one stage to another, 

 when it touches a tactile hair ; these serve to signal its 

 advance. Such hairs are consequently placed more 

 closely on the arch, where the ^g'g is caught between 

 the two spiculae ; and by their means the fly is guided 

 exactly to where the egg-body is to be found. When 

 the ^%% has reached the proper point, the egg-stalk is 

 apparently seized by a rapid to and fro movement of 

 the two spiculae. Thus all the time the egg is being 

 pushed down to the ovipositor, there is a provision for 

 keeping the fly informed of its progress by sensation. 

 There are also on the seta, particularly towards its 

 point, organs of sensation, not in the form of hairs but 

 of slight projections of the chitinous membrane. There 

 are besides, in some hymenoptera {Ptatygaster\ perfect 

 hairs on the point of the ovipositor. The sensory 

 branches of the nerve, which is contained in the central 



^ In no other order of insects is the abdominal ganghon so power- 

 fully developed as in the hymenoptera ; this is due to the fact 

 that it has to undertake the innerv'ation of the complicated piercing 

 apparatus. 



