Page:EB1911 - Volume 13.djvu/442

Rh stomodaeum. Folsom has detected in the embryo of Anurida a pair of ganglia (fig. 18, 5) belonging to the maxillular (or superlingual) segment, thus establishing seven sets of cephalic ganglia, and supporting his view as to the composition of the head.

Air-tubes.—The air-tubes, like the food-canal, are formed by invaginations of the ectoderm, which arise close to the developing appendages, the rudimentary spiracles appearing soon after the budding limbs. The pits leading from these lengthen into tubes, and undergo repeated branching as development proceeds.

Dorsal Closure.—The germ band evidently marks the ventral aspect of the developing insect, whose body must be completed by the extension of the embryo so as to enclose the yolk dorsally. The method of this dorsal closure varies in different insects. In the Colorado beetle (Doryphora), whose development has been studied by W. M. Wheeler, the amnion is ruptured and turned back from covering the germ band, enclosing the yolk dorsally and becoming finally absorbed, as the ectoderm of the germ band itself spreads to form the dorsal wall. In some midges and in caddis-flies the serosa becomes ruptured and absorbed, while the germ band, still clothed with the amnion, grows around the yolk. In moths and certain saw-flies there is no rupture of the membranes; the Russian zoologists Tichomirov and Kovalevsky have described the growth of both amnion and embryonic ectoderm around the yolk, the embryo being thus completely enclosed until hatching time by both amnion and serosa. V. Graber has described a similar method of dorsal closure in the saw-fly Hylotoma.

Mesoderm, Coelom and Blood-System.—From the mesoderm most of the organs of the body—muscular, circulatory, reproductive—take their origin. The mass of cells undergoes segmentation corresponding with the outer segmentation of the embryo, and a pair of cavities—the coelomic pouches (fig. 16, M)—are formed in each segment. Each coelomic pouch—as traced by Heymons in his study on the development of the cockroach (Phyllodromia)—divides into three parts, of which the most dorsal contains the primitive germ-cells, the median disappears, and the ventral loses its boundaries as it becomes filled up with the growing fat body (fig. 19). This latter, as well as the heart and the walls of the blood spaces, arises by the modification of mesodermal cells, and the body cavity is formed by the enlargement and coalescence of the blood channels and by the splitting of the fat body. It is therefore a haemocoel, the coelom of the developed insect being represented only by the cavities of the genital glands and their ducts.

Reproductive Organs.—In the cockroach embryo, before the segmentation of the germ-band has begun, the primitive germ-cells can be recognized at the hinder end of the mesoderm, from whose ordinary cells they can be distinguished by their larger size. At a later stage further germ-cells arise from the epithelium of the coelomic pouches from the second to the seventh abdominal segments, and become surrounded by other mesoderm cells which form the ovarian or testicular tubes and ducts (fig. 19, g). In the male of Phyllodromia the rudiment of a vestigial ovary becomes separated from the developing testis, indicating perhaps an originally hermaphrodite condition. An exceedingly early differentiation of the primitive germ-cells occurs in certain Diptera. E. Metchnikoff observed (1866) in the development of the parthenogenetic eggs produced by the precocious larva of the gall-midge Cecidomyia that a large “polar-cell” appeared at one extremity during the primitive cell-segmentation. This by successive divisions forms a group of four to eight cells, which subsequently pass through the blastoderm, and dividing into two groups become symmetrically arranged and surrounded by the rudiments of the ovarian tubes. E. G. Balbiani and R. Ritter (1890) have since observed a similar early origin for the germ-cells in the midge Chironomus and in the Aphidae.

The paired oviducts and vasa deferentia are, as we have seen, mesodermal in origin. The median vagina, spermatheca and ejaculatory duct are, on the other hand, formed by ectodermal inpushings. The classical researches of J. A. Palmén (1884) on these ducts have shown that in may-flies and in female earwigs the paired mesodermal ducts open directly to the exterior, while in male earwigs there is a single mesodermal duct, due either to the coalescence of the two or to the suppression of one. In the absence of the external ectodermal ducts usual in winged insects, these two groups resemble therefore the primitive Aptera. The presence of rudiments of the genital ducts of both sexes in the embryo of either sex is interesting and suggestive. The ejaculatory duct which opens on the ninth abdominal sternum in the adult male arises in the tenth abdominal embryonic segment and subsequently moves forward.

After hatching or birth an insect undergoes a process of growth and change until the adult condition is reached. The varied details of this post-embryonic development furnish some of the most interesting facts and problems to the students of the Hexapoda. Wingless insects, such as spring-tails and lice, make their appearance in the form of miniature adults. Some winged insects—cockroaches, bugs (fig. 20) and earwigs, for example—when young closely resemble their parents, except for the absence of wings. On the other hand, we find in the vast majority of the Hexapoda a very marked difference between the perfect insect (imago) and the young animal when newly hatched and for some time after hatching. From the moth’s egg comes a crawling caterpillar (fig. 21, c), from the fly’s a legless maggot (fig. 25, a). Such a young insect is a larva—a term used by zoologists for young animals generally that are decidedly unlike their parents. It is obvious that the hatching of the young as a larva necessitates