100 ARACHNIDA. 



The Larva. The hexapod Acarid larva shows a strong general 

 resemblance in structure to the adult.* This is especially the case 

 when the manner of life of the larva is the same as that of the 

 imago, as, for example, in the Halacaridae (Halacarus spinifer> 

 Lohhann, No. 92). The same resemblance occurs in many Trom- 

 bidiidae, while in other members of this family the larvae differ in 

 structure from the adult. The larva is chiefly distinguished from 

 the adult by the more primitive character of its organisation,, 

 especially of the segmentation of the body. In the embryo of 

 Tyroglyphus siro there is, at the posterior part of the cephalo-thorax, 

 a distinct division into three segments, which can still be found in 

 the larva (Claparede). These segments correspond to the pairs 

 of limbs. In the larva of Trombidium, the cephalo-thorax shows 

 six segments corresponding to the pairs of limbs (Henking). Seg- 

 mentation even appears in the abdomen in Trombidium (Fig. 52} 

 and the Oribatidae (Henking, Michael, No. 97). This part of the 

 body is then larger than in the Gamasus larva illustrated in Fig. 54. 

 The abdominal segmentation may also be retained in some cases, 

 like that of Alycus roseus described by Kramer (No. 89). In this 

 Acarid, the abdomen of the adult female is marked out into seven 

 distinct segments, and segmentation can also be recognised in the 

 thorax. Segmentation of both cephalo-thorax and abdomen seems- 

 also to take place in members of the genus Tarsonymus (Dendroptus, 

 Kramer, Nos. 87 and 88). 



According to Haller (No. 83) and Oudemans (No. 11), the Acarina possess- 

 from three to four pairs of mouth-parts, a greater number than has hitherto 

 been assumed. Ontogenetically, however, this view is not supported, as only 

 the rudiments of the two well-known pairs of mouth-parts (chelicerae and 

 pedipalps) are recognisable (Figs. 51-53). It is now generally admitted that 

 Haller was in error in his attempt to prove that the Acarina had two pairs of 

 maxillae like the Insecta. 



The frequent appearance of a furrow between the second and third legs has 

 led many observers to conclude that the part lying behind this furrow belongs to 

 the abdomen, and that the two posterior pairs of legs are abdominal appendages. 

 There is, however, considerable disagreement on this point, which further is not 

 supported by ontogeny. Henking in particular states that this does not hold 

 good for Trombidium, nor, in his opinion, for any Acarids. These two posterior 

 legs are attached to what is commonly called the abdomen ; if the term be 

 incorrectly applied, then it appears that the Acarina have no abdomen. 



The mouth-parts of the larva already show the character of those 

 of the adult, i.e., the chelicerae,! with the basal parts of the 

 pedipalps, unite to form a proboscis (Fig. 53 B). The greater part 



* [See footnote, p. 97.— Ed.] + [See footnote, p. 97. — Ed.] 



