Fig. 196. Three stages in the development of the Scorpion. The embryos are represented as if seen extended on a plane.
(After Metschnikoff.)
ch. cheliceræ; pd. pedipalpi; p1-p4. ambulatory appendages; pe. pecten; st. stigmata; ab. post abdomen (tail).
Pseudoscorpionidæ. The development of Chelifer has been investigated by Metschnikoff (436), and although (except that it is provided with tracheæ instead of pulmonary sacks) it might be supposed to be closely related to Scorpio, yet in its development is strikingly different.
The eggs after being laid are carried by the female attached to the first segment of the abdomen. The segmentation (vide p. [93]) is intermediate between the types of complete and superficial segmentation. The ovum, mainly formed of food-yolk, divides into two, four, and eight equal segments ([fig. 197] A). There then appear one or more clear segments on the surface of these, and finally a complete layer of cells is formed round the central yolk spheres ([fig. 197] B), which latter subsequently agglomerate into a central mass. The superficial cells form what may be called a blastoderm, which soon becomes divided into two layers ([fig. 197] C). There now appears a single pair of appendages (the pedipalpi) ([fig. 198] A, pd), while at the same time the front end of the embryo grows out into a remarkable proboscis-like prominence—a temporary upper lip (concealed in the figure behind the pedipalpus), and the abdomen (ab) becomes bent forwards towards the ventral surface. In this very rudimentary condition, after undergoing an ecdysis, the larva is hatched, although it still remains attached to its parent. After hatching it grows rapidly, and becomes filled with a peculiar transparent material. The first pair of ambulatory appendages is formed behind the pedipalpi and then the three succeeding pairs, while at the same time the cheliceræ appear as small rudiments in front. External signs of segmentation have not yet appeared, but about this period the nervous system is formed. The supra-œsophageal ganglia are especially distinct, and provided with a central cavity, probably formed by an invagination, as in other Arachnida. In the succeeding stages ([fig. 198] B) four provisional pairs of appendages (shewn as small knobs at ab) appear behind the ambulatory feet. The abdomen is bent forwards so as to reach almost to the pedipalpi. In the later stages ([fig. 198] C) the adult form is gradually attained. The enormous upper lip persists for some time, but subsequently atrophies and is replaced by a normal labrum. The appendages behind the ambulatory feet atrophy, and the tail is gradually bent back into its final position. The segmentation and the gradual growth of the limbs do not call for special description, and the formation of the organs, so far as is known, agrees with other types.
Fig. 197. Segmentation and formation of the blastoderm in Chelifer.
(After Metschnikoff.)
In A the ovum is divided into a number of separate segments. In B a number of small cells have appeared (bl) which form a blastoderm enveloping the large yolk spheres. In C the blastoderm has become divided into two layers.
The segmentation of Chthonius is apparently similar to that of Chelifer (Stecker, No. [437]).
Phalangidæ. Our knowledge of the development of Phalangium is unfortunately confined to the later stages (Balbiani, No. [438]). These stages do not appear however to differ very greatly from those of true Spiders.