630 ARACHNIDA. 



fig. 59, 4) and are usually of the same form. Thus one cepha- 

 lic segment of insects is permanently retained in the thorax 

 among the Arachnids, whereas we have seen in the embryo of 

 the dragon-fly (Figs. 59, 61, 4) it assumes an intermediate 

 position between the head and thorax, the remaining anterior 

 part of the head being clearly separated by a deep suture. In 

 Fig. 59, we see the labial palpi (4) grouped with the three pairs 

 of legs ; a position permanent in the Arachnida. The dragon- 

 fly, at the period represented by Fig. 59, p. 57, may be legiti- 

 mately compared with the scorpion, especially Cyclopthalmus, 

 from the coal measures. 



While, as Blackwall states, nothing is known with certainty 

 concerning the organs of smell and hearing in spiders, Mr. 

 R. Beck "■suggests that spiders are capable of distinguishing 

 sounds to some extent by means of very delicate waving hairs 

 which are found on the upper surfaces of their legs. During 

 life they move at their peculiarly cup-shaped bases, with the 

 least motion of the atmosphere, but are immovable after 

 death. It is well known that sound is due to vibrations which 

 are generally convej^ed by undulations of the air ; now I am 

 perfectly satisfied that if these undulations are of a certain 

 character the hairs I am alluding to, upon the spider's leg, will 

 move, and I wish you particularly to notice that they are of 

 diflferent lengths, so that some might move whilst others would 

 not, and also that the longest is at the extremity of the leg, 

 and therefore can receive an undulation which might die away 

 higher up. I may just mention that there is a group of these 

 peculiar hairs on the flea. The legs of a spider are most sen- 

 sitive organs of feeling, if they do not also embrace those of 

 hearing." (Entomologist, London, l-SGG, iii, p. 24G.) 



The four thoracic feet have seven joints, and it is probable 

 that the two basal joints homologize with the coxa and tro- 

 chantine of insects, in which the two joints are retracted, side 

 by side, and closely fused together. The tergal part of the 

 thoracic segments is large, overlapping the pleural, while the 

 sternum is a rather large, broad breast-plate. The abdomen 

 is generalh^ somewhat spherical, and in but few instances is it 

 drawn out and the rings well developed, as in the scorpion. 

 In the mites it is fused closely with the cephalothorax. 



