Fig. 201. Two late stages in the development of Agelena labyrinthica.
A. Embryo from the side at the stage when there is a large ventral protuberance of yolk. The angle between the line of insertion of the permanent and provisional appendages shews the extent of the ventral flexure.
B. Embryo nearly ready to be hatched. The abdomen which has not quite acquired its permanent form is seen to be pressed against the ventral side of the thorax.
pr.l. procephalic lobe; pd. pedipalpi; ch. cheliceræ; c.l. caudal lobe; pr.p. provisional appendages.
The general character of the changes which take place during this period in the development is shewn in [fig. 201] A and B representing two stages in it. In the first of these stages there is no constriction between the future thorax and abdomen. The four pairs of provisional appendages exhibit no signs of atrophy; and the extent of the ventral flexure is shewn by the angle formed between the line of their insertion and that of the appendages in front. The yolk has enormously distended the integument between the two halves of the ventral plate, as is illustrated by the fact that, at a somewhat earlier stage than that figured, the limbs cross each other in the median ventral line, while at this stage they do not nearly meet. The limbs have acquired their full complement of joints, and the pedipalpi bear a cutting blade on their basal joint.
The dorsal surface between the prominent caudal lobe and the procephalic lobes forms more than a semicircle. The terga are fully established, and the boundaries between them, especially in the abdomen, are indicated by transverse markings. A large lower lip now bounds the stomodæum, and the upper lip has somewhat atrophied. In the later stage ([fig. 201] B) the greater part of the yolk has passed into the abdomen, which is now to some extent constricted off from the cephalo-thorax. The appendages of the four anterior abdominal somites have disappeared, and the caudal lobe has become very small. In front of it are placed two pairs of spinning mammillæ. A delicate cuticle has become established, which is very soon moulted.
Acarina. The development of the Acarina, which has been mainly investigated by Claparède (No. [446]), is chiefly remarkable from the frequent occurrence of several larval forms following each other after successive ecdyses. The segmentation (vide p. [116]) ends in the formation of a blastoderm of a single layer of cells enclosing a central yolk mass.
A ventral plate is soon formed as a thickening of the blastoderm, in which an indistinct segmentation becomes early observable. In Myobia, which is parasitic on the common mouse, the ventral plate becomes divided by five constrictions into six segments ([fig. 202] A), from the five anterior of which paired appendages very soon grow out ([fig. 202] B). The appendages are the cheliceræ (ch) and pedipalpi (pd) and the first three pairs of limbs (p1-p3). On the dorsal side of the cheliceræ a thickened prominence of the ventral plate appears to correspond to the procephalic lobes of other Arachnida. The part of the body behind the five primitive appendage-bearing segments appears to become divided into at least two segments. In other mites the same appendages are formed as in Myobia, but the preceding segmentation of the ventral plate is not always very obvious.
In Myobia two moultings take place while the embryo is still within the primitive egg-shell. The first of these is accompanied by the apparently total disappearance of the three pediform appendages, and the complete coalescence of the two gnathiform appendages into a proboscis ([fig. 202] C). The feet next grow out again, and a second ecdysis then takes place. The embryo becomes thus inclosed within three successive membranes, viz. the original egg-shell and two cuticular membranes ([fig. 202] D). After the second ecdysis the appendages assume their final form, and the embryo leaves the egg as an hexapodous larva. The fourth pair of appendages is acquired by a post-embryonic metamorphosis. From the proboscis are formed the rudimentary palpi of the second pair of appendages, and two elongated needles representing the cheliceræ.
