THE FORMATION OF THE NYMPH. 103 



the larva, Henking is disposed to regard as a stigma.* The aperture, 

 by means of the funnel, introduces air into the embryo. When 

 ecdysis takes place, the funnel naturally becomes detached from 

 the stigma (Fig. 52). Such "primitive tracheae" are found in 

 corresponding positions in the larvae of other Acarina as well. 

 More than half the Acarina are without eyes in all stages. Some 

 of the free-living larvae possess one or two pair of eyes, lying at 

 the anterior margin of the brain. Since this latter has shifted far 

 back, the eyes also lie far back above the bases of the second pair 

 of limbs {Trombidium, Atax, Tetranyehus). The middle part of the 

 cephalo-thorax has thus shifted forward beneath the anterior part. 



The Nymph. After the larva has remained in the form described 

 for a longer or shorter time, according to its manner of life, further 

 changes take place. In Atax Bonzi, the larva bores into the branchial 

 tissue of its Lamellibranch host, and loses its capacity for movement. 

 The soft parts now become detached from the chitinous cuticle as in 

 a typical ecdysis ; the limbs draw back from their chitinous sheaths 

 and become nearly absorbed, remaining however as small knobs. 

 The chitinous cuticle itself swells up by the absorption of water, 

 and the body, which has secreted a fresh . cuticular covering, swims 

 about as an almost spherical body inside the detached shell. This 

 process resembles to a great extent that described for Myobia during 

 the formation of the larva. The limbs then grow out again,! a 

 fourth pair being added. The larva in this form is known as the 

 nymph, and resembles the adult in its shape and also in the number 

 of its limbs, but does not quite equal it in perfection or in sexual 

 maturity. It commences its free life by breaking through the larval 

 integument. 



The new pair of limbs is always the fourth, at least this has been established 

 in several forms, e.g., Trombidium (Henking), Ixodes, Tarsonymus {Dendroptus 

 of Kramer) ; in aquatic forms, according to Kramer (No. 87), especially in 

 the genus Ncsaea, one of the first two pairs of limbs was newly added. 

 Lohmann observed that in the Halacaridae the second pair of limbs developed 

 very slowly, although he also regards the fourth pair as the one newly added. 

 Oudemans (Xo. 11), on the contrary, lays special stress on the fact that, in the 

 larva of the Oribatidae, the new pair of limbs is intercalated between the 

 second and third of those already present. J 



The transition from larva to nymph in other forms is not so simple 

 as in the case described. The six-limbed larva of Rhyncholophus 



* [It is very doubtful if this be a stigma ; analogy with other families would 

 point to a different conclusion. — Ed.] 

 t [See footnote, p. 97. — En.] 

 t [Both Kramer and Otdemans appear to be in error on these points. — Ed.] 



