240 THE CAUSES WHICH PROPAGATE 



ganglia have their usual position, and the first abdominal knot 

 is close to the metathoraeic. The ensuing ganglion is situated 

 in the centre o£ the first abdominal segment, and the next in the 

 centre of the third. The fourth and fifth ganglia are similarly 

 placed in the fifth and eighth rings. Of these nervous centres 

 the metathoraeic fu.rnishes branches to the first ring and the 

 greater part of the second, sending special nerves to the tympanal 

 muscles, the mechanism for closing the spiracles, and the dorsal 

 muscles of the second ring. Similarly here, as in the grass- 

 hoppers, a nerve (Plate VI., Fig. 9, n) proceeding to the 

 tympanum parts into two principal veins, one of which runs to 

 the closing muscle of the fourth spiracle, and the other directly 

 to thedrumskin; but this latter, instead of there forming a definite 

 termination, ramifies over the tympanum. So that, on the whole, 

 we find in certain crickets organs with much analogy to the 

 abdominal auditory structures of the grasshoppers ; but whether 

 they have actually effective supplementary ears must be left to 

 future investigators to decide. Otherwise, in tunnelling insects 

 such as constitute this group, auditory membranes posteriorly 

 directed appear quite consonant with the habits. 



If we turn now to the tibial structures of the crickets, we find 

 that these, although they were noticed by Colonel Goureau, who 

 also surmises their function, have their first investigator in Von 

 Siebold, who, having recognised an organ in the Acrididse 

 structurally adapted inwardly and outwardly to receive the 

 aerial sound-waves and conduct them to a percipient nerve, 

 turned his attention to the Locustidse. Having failed here to 

 find a counterpart in the required situation, he on further exami- 

 nation became struck by the fundibular tracheal openings 

 (Plate II., Fig. 2e) present in this group beneath the hinder 

 edge of the prothorax, a circumstance which was followed by 

 his recognition of the double clefts in the fore tibise, which on 

 examination proved in direct communication with these (Fig. 2, 

 a, m) , the two parts affording him the required analogies of the 

 acridiideous organs. 



Behind the posterior edge of the prothorax, then, according to 

 Von Siebold, lay on either side two tracheal openings — one small 

 and closed with a valvular lip, the other situated just above, a 

 large funnel which projects with a singularly broad opening 



