132 Nmnber and Position of Spiracles in Arachnida 



tracheffi branch off from its extreme end, and from that side which turns inwards and 

 downwards. These minor tubes do not ramify and their width is uniform in the whole of 

 their length, but they are not all equally wide, the diameter of the widest being quite 

 twice that of the narrowest. Those tubes which start from the end of the trunk go exclu- 

 sively to the cephalothorax and the limbs, whilst those which rise from the side go to the 

 abdomen, at least for the most part. The chitinous membrane of the basal half of the 

 trunk is furnished with numerous rather short and very narrow, irregular, and not very 

 distinct thicker lines, which are mostly transverse, and not connected so as to form a network ; 

 the membrane of the apical half of the trunk and along that side where the minor tubes 

 take their rise, is closely sprinkled with thickened points so as to produce a finely granular 

 appearance. In the membrane of the minor tubes we have not been able to discover any 

 vestiges of any kind of thickening. 



The number and position of spiracles in Arachnida generally is not a little varying. 

 Special organs of respiration are wanting in Palpigradi (Koenenia) and in Acari atracheati. 

 The abdomen alone carries spiracles in Scorpiones (four pairs), in Chelonethi (two pairs), in 

 Pedipalpi (two pairs, generally, but in Tartarides, according to our own investigations (b), only 

 one pair), and in Araueas (two pairs, of which the hindmost pair, in Aranese tristictas Bertk., 

 is fused into one spiracle); furthermore in the majority of Opiliones (one pair), and amongst 

 Acari in Ixodidee (one pair situated behind the fourth pair of coxa^). Solifugas have spiracles 

 both on the abdomen and on the cephalothorax (the head), whilst the family Phalangioidae, 

 amongst Opiliones Palpatores, have two spiracles on each walking limb, besides the ordinary 

 pair on the abdomen^ It was hitherto only within the order of Acari that examples were 

 known of spiracles existing only on the cephalothorax'-. From the foregoing it will be seen that 

 this is the case in Ricinulei. We have mentioned the position of the spiracles in Arachnida 

 generally, partly in order to point out how great the differences are in this respect within 

 the class, but partly also in order to show that, apart from Acari, which in .so many 

 respects are polymorphous, the arrangement of the spiracles is almost constant within each 

 order by itself. The difference existing between Ricinulei and Pedipalpi in this matter must 

 therefore be looked upon as rather essential. — McLeod (p. 30) states : " Enfin, la position des 

 stigmates indique ^galement une nature differente : chez les iusectes et les myritfpodes, les 

 stigmates sont situes au-dessus de I'insertion des membres, du cote dorsal, tandis que les 

 stigmates des Arachnides sont, comme les orifices des poumons, situes a la face ventrale." As 

 the treatises of McLeod on the organs of respiration in Arachnida are much consulted we 

 will not omit to point out that this statement is untenable. Because even if the spiracles 

 in Insects could be justly described as dorsal — which is very doubtful — they cannot in a 

 general way be said to be ventral in Arachnida. The pair of spiracles situated on the 

 cephalothorax of Solifugae cannot be described as occupying a ventral position as regards the 

 limbs ; in Ricinulei the solitary pair of spiracles is decidedly placed above the limbs ; nor 

 is the position of the only pair of spiracles occurring in Ixodidie a ventral one, as they are 



1 See Hansen {b, pp. 198—204). According to Loman as to the state of things in most of these animals rest on 



(e) the spiracles on the walking limbs are not developed in the authority of other writers, and chiefly that of Kramer 



quite young animals, but make their appearance in a later (p. 219), Michael {a, p. 50), and With (p. l-SU). According to 



stage of development. these authors the spiracles (whether one or more pairs) are 



- This seems to be the case in Acari tracheati, with the placed exclusively on the cephalothorax in Prostigmata 



■exception of Ixodida; and Notostigmata. As we have not Kram., Oribatidse, Gamasida? and Myohin. 

 ourselves made any extensive study of Acari, our statements 



