286 Herr J. Wagner on the 



Acarina to the ancestral forms is closer than that of tlie adults. 

 There is, however, as is well known, a whole series of forms 

 which are destitute of trachete in the adult condition also 

 {Acarina atracheata, Kramer, No, 37, p. 218) ; yet tliese forms 

 cannot be regarded as the more primitive, since the majority 

 of their number (the family Tyroglyphidaj excepted) are para- 

 sites. As a matter of fact, parasitism more than anything 

 else has affected the bodily size of the Acarina : all Acarina 

 atracheata are very small, and only exceptionally attain a 

 length of 1 millim.* In all probability the insignificant size 

 of these Acarina occasioned the loss of the tracheas. The 

 same conditions are found in the case of the larval Acarina, 

 and we are tempted to assume that in the Acarina the forma- 

 tion of the tracheae has been cenogenetically transferred to 

 the postembryonic period, while they originally possessed 

 tracheae even in the earliest larval stage. In consideration 

 of yet other indications of the secondary character of Acarine 

 larva3, such an assumption appears to me to be more credible 

 than the former. 



When it is desired to point out the relationship between 

 the x\rachnids and Limidus, we proceed inter alia from the 

 comparison of the lungs of the Scorpion and the branchial 

 feet of LimiduSj and base our view upon observations on the 

 development of the lungs of the Scorpion and of the Arane^e ; 

 the great similarity in structure between the gills of Limulm 

 and the lungs of the Scorpion has been demonstrated by 

 MacLeod (No. 42 h), while the difference in development 

 between the lungs of Arane^e and the typical trachege has 

 been pointed out by Berteaux (No. 8). The trachege in the 

 Araneas arise by means of a further modification of the lung- 

 sacs. While such an explanation of the origin of the tracheae 

 meets the case in certain orders of Arachnids, it is otherwise 

 with respect to the trachea of the Acarina (partly also of the 

 Solifugaj), the stigmata of which are situated upon the cepha- 

 lothorax (the position of the stigmata in the Ixodidas is 

 somewhat diflScult to determine ; yet I consider that we may 

 reasonably assume that this family forms no exception to the 

 general rule). The trachege of the Acarina have certainly 

 not been developed by means of the metamorphosis of any 

 appendages ; yet in considerations as to their origin the first 

 question that forces itself upon us is wliether the common 



* An interesting exception is constituted by, e. g., Tyroglyphus myco- 

 phagits, Meg., tlie females of which, according to Meguin, attain a size of 

 1 to 1-25 millim., while according to Moniez (No. 47, pp. 590-591) under 

 favourable conditions of life they exceed 2 millim. in size, and are then 

 capable of producing living young. 



