THE FORMATION OF THE LARVAL INTEGUMENTS. 97 



Acarina show great resemblance to those of the Araneae (Figs. 51 

 and 57 A). The chelicerae and pedipalps unite to form the proboscis 

 (Fig. 53).* The abdomen (in Atax) now decidedly preponderates 

 over the anterior part of the body (Fig. 53). There are only three 

 pairs of limbs when the embryo breaks through its envelopes and 

 begins free life (Figs. 51-53, p x -p^). We thus find, in the Acarina, 

 a larval stage with only three pairs of limbs, as distinguished from 

 the four pairs of the nymph and of the adult, which, in other 

 points of both outer and inner organisation, the embryo greatly 

 resembles.t 



2. The Formation of the Larval Integuments and the 

 Further Course of Development. 



It was mentioned that, in many Acarina, e.g., Atax, the embryo 

 casts off a cuticular integument at an early stage when the limbs 

 have not yet developed or are only indicated. Claparede's deut- 

 ovum is thus produced, the embryo within the egg-shell thus 

 becoming enclosed in a second envelope (Fig. 51). The resemblance 

 of the "deutovum" with the embryo enclosed in it to an intact egg 

 is increased by the fact that, after casting off the primary egg-shell 

 (eh), the embryo undergoes further changes in its external form within 

 the deutovum. In Trombidlum and Myobia this cuticular membrane 

 is cast only after the rudiments of the limbs have appeared (Fig. 52). 

 In Trombidium, this membrane is provided with appendages which 

 surround the limbs like sheaths (Hen king), but this is not the case 

 in Myobia. Here the limbs form in the usual way (Fig. 50 B), but 

 when they have grown to a considerable length they become applied 

 to the ventral surface of the body, and gradually become flattened to 

 such a degree as hardly to project from the surface of the body. 

 The whole embryo is once more oval and apparently devoid of 

 appendages (Fig. 50 C). At this stage a cuticular membrane becomes 

 detached from the embryo, bearing near its antero-dorsal extremity 

 (in the nuchal region, according to Claparede) a tooth-like structure, 

 composed of two thin chitinous processes closely applied to one 

 another. This structure is not well depicted in Fig. 50 C and D, 



* [This is true of the forms described by Henking, but by no means holds 

 good for all the Acarina, in the majority of which the chelicerae remain as 

 perfectly distinct and movable organs. — Ed.] 



t [In the Phytoptidae the adult has only two pairs of legs. The larvae and 

 nymphs do not always resemble the adults in other respects, for instance, in the 

 Oribatidae, they differ essentially in external appearance, and the adult has a 

 well-developed tracheal system which is entirely wanting in the larva. — Ed.] 



H 



