106 



ARACHNIDA. 



is attained by further growth and the complete development of the 



genital organs. 



As already mentioned, histolytical processes take place during the trans- 

 formation of the nymph into the imago. These do not affect all the organs, 



the genital organs heing entirely 

 unaffected hy them. The tracheal 

 system of the nymph, the stigma 

 of which lies at the hase of the 

 chelicerae, does not pass over to 

 the adult, but the tracheal tubes 

 remain in the cast-off larval in- 

 tegument (Henking). The "primi- 

 tive tracheae, "mentioned above, are 

 entirely unconnected with the adult 

 tracheal system. 



Summary. Deviations 

 from the usual course of 

 development. The develop- 

 ment of the Acarid, from the 

 egg to the adult, consists of a 

 succession of larval and pupal 

 stages. Even within the egg 

 a stage occurs (the deutovum) 

 which greatly resembles the 

 later pupal stages. This leads, 

 after a moult, to the free larva 

 with six limbs, which passes 

 into a resting stage, and through 

 it develops either directly or 

 through the development of a 

 pupal integument into the 

 eight -limbed nymph. This 

 also passes into a resting stage, 

 casts off the nymph-integument, and after the formation of another 

 pupal envelope gives rise to the young Acarid, which resembles in 

 form the sexually mature adult.* 



The above is merely a general account of the course of development in the 

 Acarina, and is not by any means an exhaustive description, since individual 

 families, genera, and species differ in one point or another. A really complete 

 account would be far beyond the scope of this book, not only because of the 



* [AH stages not sexually mature are considered nymphal; there are generally 

 several nymphal ecdyses ; in the Oribatidae two such ecdyses occur, together 

 with a final one, in which the nymph passes into the sexually mature 

 Acarid. — Ed.] 



Fig. 56. — Nymph of Trombidium fuliginosum in 

 the stage of the development of the pupa and 

 imago (after Henking). a, anus; ch, chelicerae 

 of the imago ; P^P^, limbs ; Pcd, pedipalps of 

 the imago ; Pj-i'i, limbs of the nymph (partly 

 broken off) ; r, proboscis (chelicerae and pedi- 

 palps) of the nymph; zh, intermediate integu- 

 ment. 



