ARTHROPODA : INSECT A. 497 



tinguishably fused, seldom clearly separate, the line of division being usually indi- 

 cated by a furrow. Each principal division of the body provided with two well- 

 developed pairs of limbs. There are three pairs of jaws with a rudimentary upper 

 lip and large under lip. The first pair of maxillae palpate, the second pair often 

 rudimentary. The larva is hexapodous, the post-embryonal development frequently 

 interrupted by a Deutovum stage.' Haller makes two orders, (i) ' Acarina 

 atracheata, Mites devoid of tracheae, respiring through the integument ; ' and (2) 

 ' A. tracheata, Mites provided with tracheae at least in the adult condition.' 



It remains to be seen whether Haller's conclusions meet with acceptance and 

 confirmation. The cuticular membranes cast off by some Mites in the course of 

 development, and characteristic of the Deutovum and Tritovum stages, are by no 

 means always developed. But if the mouth-parts really consist of three pairs, 

 mandibles, first and second maxillae, and a palpate lower lip which represents a 

 fourth pair of metamorphosed appendages, together with four pairs of limbs, then a 

 Mite possesses two pairs of appendages not present in the adult Spider. But it 

 should be borne in mind that there are in Spiders aborted embryonic limbs behind 

 the last pair of legs, and the Scorpion actually retains two additional pairs in the 

 genital operculum and pectinate appendages. See table, pp. 174-5. The Acarina 

 might be regarded as a group of Arachnida with two additional pairs of limbs con- 

 verted into mouth-parts, just as in the branchiate Arachnida there are five pairs of 

 limbs in relation with the mouth in Limulus, four pairs in Eurypterina and Trilobita. 

 Oudemans states that the fourth pair of ambulatory limbs is intercalated in the 

 Oribatidae, between the first and second pairs of limbs of the hexapod larva. Such 

 a fact rather points to the conclusion that the hexapod stage has been secondarily 

 acquired by the temporary suppression of a somite. 



Affinities of Arthropoda, Kingsley, Q. J. M. xxv. 1885, p. 556 et seqq. ; 

 Oudemans, Tijdschrift der Nederland. Dierk. Vereen. (2), i. 1885, p. 37. 



Phylogeny of Arthropoda, Balfour, Comp. Embryology, i. p. 451. 



Acaroidea, Haller, Z. A. iv. 1881, p. 380. 



Eyes of Arthropoda, Patten, Mitth. Zool. Stat. Naples, vi. 1886 ; Ray 

 Lankester and A. G. Bourne, Q. J. M. xxiii. 1883. 



CLASS INSECTA. 



(Hexapoda^ 



Tr ache ate Arthropoda with the body divided into three regions, a 

 head, thorax, and abdomen. The head carries a pair of antennae and three 

 pairs of oral appendages, the thorax three pairs of jointed locomotor appendages, 

 and in most instances one or two pairs of wings. Abdominal limbs are rarely 

 present. 



The head shows no trace of segmentation and is sharply marked off 

 from the thorax, to which it is moveably connected by a neck in the 

 imago. It is also generally distinct in the larva except in some Diptera. 

 The thorax consists of a pro-, meso-, and meta-thorax. The pro-thorax 

 may be free, e. g. Coleoptera, many Rhynchota, but as a rule the three 



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