HOW AND WHY SCORPIONS HISS. 



23 



features pointed out, the notes in all the specimens examined are alike, 

 being thin, flat, leaf-like, and finely striate. The edges, when entire, 

 are evenly convex, though not infrequently they present a ragged 

 appearance as if frayed from rough usage. That these notes are 

 nothing but modified bristles there can be little doubt, though since 

 they now occupy an isolated area practically free from hairs, the direct 

 evidence of their origin is not so clear as it is in the analogous cases 

 presented by the species of Scorpio and by the mygalomorphous 

 spiders. 



Of all the species of Opisthophthalmus contained in the British 

 Museum two only, namely 0. zvahlbevgi and O. pallidimanus, show no 

 traces of this instrument. But in these, as in the other species of the 

 genus, the upper surface of the basal segment of the mandible is 

 raised at its distal end into a prominence thickly studded with 

 bristles (Fig. 4, A and B) ; and when examined under a high power 

 these bristles may be seen to be modified in exactly the same way as 



C. Z*^^^. ^- ^^ 



F, -:^^^^L 



Fig. 4. — Stridulating Organs of Opisthophthalmus. 

 A, Inner surface cf mandible of 0. glabvifrons, with three notes and bristly 

 prominence on the basal segment. B, Basal segment, with notes and bristly promi- 

 nence, in 0. carinatus. C, Keyboard of 0. glabvifrons. D, One of the notes of 0. 

 carinatus. E, One of the notes of 0. granifrons. F, Some of the notes from the tuft 

 of bristles of 0. capensts. 



are those that constitute the notes in the species of Scorpio — that is to 

 say, the base is thickened and compressed, while the slender terminal 

 portion is bent over at right angles as represented in Fig. 4, F. From 

 what is known of the function of the similarly-constructed bristles in 

 Scorpio swammerdami, for instance, it cannot reasonably be doubted 

 that these bristles subserve the same purpose of producing sound in 

 Opisthophthalmus, and in the absence of any specially-constructed 

 scraper we may conclude that they are thrown into a state of vibration 

 by scraping against the front edge of the carapace, as the mandible is 

 forcibly withdrawn beneath it. There are thus two distinct stridu- 

 lating organs within the limits of this genus. In some of the species, 

 indeed, the two organs exist side by side, as may easily be seen by 

 examining the mandible of 0. capensis, the species that occurs 

 abundantly in the vicinity of Cape Town. 



Apart from this new organ, the most interesting feature con- 

 nected with Opisthophthalmus is the characteristic from which the 

 genus derives its name, namely, the backward position of the median 



