GENERAL CONSIDERATIONS. 109 



It is said that several forms capable of reproduction may occur in this way in 

 one and the same species ; Gamasus tardus, for instance, has no less than five 

 such different forms, each of which might be taken for a different species 

 (Berlese).* These are evidently very complicated conditions, which are far 

 from being sufficiently understood. There is no doubt that early stages of 

 development have repeatedly been regarded as different species, as is now 

 definitely proved in the case of the well-known genus Hypopus (Megnin, 

 Nos. 94 and 95, Michael, Nos. 98 and 99). The members of this genus are 

 minute creatures with a smooth chitinous shell, convex on the dorsal side and 

 flattened on the A - entral side, covering the whole of the body. Acarids with this 

 characteristic appearance are often found on larval and on adult Insects, 

 Myriopoda, etc., and were long regarded as adults. A closer study of the course 

 of their development, however, proved that they merely represent early onto- 

 genetic stages of Tyroglyphus and related genera, which, as a result of hitherto 

 unknown circumstances, have deviated from the usual form of the nymph. 

 These variations only affect isolated individuals, and it has been attempted to 

 trace them back to unfavourable external conditions, which brought about such 

 a modification of the inner organisation (Megnin). This explanation of the 

 origin of the heteromorphic {Hypopus) forms has been disproved by Michael. 



General Considerations. 



Attempts have been made to separate the Acarina from the 

 Arachnida, and to give this group the same value as the larger 

 divisions of the Arthropoda (Arachnida, Myriopoda, Hexapoda, 

 Haller, No. 83, A. C. Oudemans, No. 11). The grounds given 

 for this classification appear to us too insufficient to deserve further 

 discussion (p. 100). It rather appears to us that in the organisation 

 and development of the Acarina there is sufficient resemblance to 

 the Arachnida to justify their being classed among these latter, in 

 accordance with the view until now commonly held. The Acarina 

 represent a group of the Arachnida with highly specialised develop- 

 ment, and are thus strongly differentiated in individual points of 

 organisation from other Arachnids. Even the course of development 

 has been influenced, and shows peculiarities which do not occur in 

 other Arachnids. The chief of these are the different consecutive 

 larval and pupal stages, and the free larval form provided with only 

 six limbs. This latter must be considered as a secondary peculiarity. 

 The best proof of this would be afforded by the appearance of a 

 fourth pair of limbs in embryonic stages, which precede the six- 

 limbed larva, if the statements made on this subject by "Winkler 

 (No. 106, cf. p. 107) should be confirmed.! 



* [According to Michael both these observations are erroneous. — Ed.] 

 t [This has been done for Ixodes by Wagner. — Ed.] 



