288 ME. H. M. BERNARD ON THE RELATION 



There is a point in the development of the Mites which has 

 received considerable attention : that is the fact that the earliest 

 larva has only three pairs of legs {cf. fig. 3 B) ; the fourth pair, 

 which is usually the most posterior, is added as a further deve- 

 lopmental process. At first sight this seems greatly to favour 

 my theory of the undeveloped state of the hind body. But until 

 more light has been thrown on Winkler's observation, that four 

 pairs of limbs develop in the embryo, one pair being reabsorbed, 

 only to be developed again in a later stage, I can hardly claim 

 it as a support to my theory. 



It is not easy to see the true bearing of Winkler's observation 

 on the origin of the Acaridae. The loss of a pair of legs is pro- 

 bably an adaptative modification to larval life*. As far as I 

 can see, the temporary appearance of the fourth pair of legs in 

 the embryo seems to show a tendency in the embryo to develop 

 into the adult Araneid, as if the full number of abdominal seg- 

 ments were about to be developed. The sudden arrest of deve- 

 lopment, probably consequent on the exhaustion of the available 

 supply of nutriment, leads to the reabsorption of the last pair of 

 limbs, the power to redevelop them being only gradually re- 

 gained after the larva has obtained for itself a fresh store of 

 nourishment. 



The tracheae also demand some attention. Their position lends 

 considerable support to our theory. The stigmata occur in many 

 diff"erent parts of the body, but almost invariably on the cephalo- 

 thorax, in some cases even so far forward as to be on the dorsal 

 surface at the base of the mandibles. In the Arachnida (ex- 

 cluding the Acaridse) the stigmata are, with one interesting 

 exception, abdominal. The Bolpugidse have stigmata on the 

 thorax. The Mites thus form a most interesting and important 

 link between the Arachnids and the Antennate Tracheata. In 

 the Araneids we have purely abdominal stigmata with tracheae, 

 both book-leaf and tubular. In the Solpugidse we have tubular 

 trachesB alone, opening on both abdomen and thorax ; and in the 

 Mites we find the stigmata almost invariably on the thorax, the 

 traehege being purely tubular. Now it is generally thought that 

 the Solpugidse are a somewhat primitive form of Arachnids; and 

 if our theory is true, that the Acarids are larval Araneids, they 

 would also be on the level of primitive forms in which we might 

 naturally expect to find a more primitive tracheal system than 



* A similar phenomenon occurs in the development of some Malacostraca, 

 cf. Lang's ' Textbook of Comparative Anatomy,' pp. 410-411. 



