Although in the adult stigmata are never found in the space between the prothorax and head[173], in the embryo and the larva tracheal invaginations may be developed in all the thoracic (and possibly in the three jaw-bearing segments) and in all the abdominal segments except the two posterior.
In the embryo of the Lepidoptera, according to Hatschek (No. [414]), there are 14 pairs of stigmata, belonging to the 14 segments of the body behind the mouth; but Tichomiroff states that Hatschek is in error in making this statement for the foremost post-oral segments. The last two segments are without stigmata. In the larvæ of Lepidoptera as well as those of many Hymenoptera, Coleoptera and Diptera, stigmata are present on all the postcephalic segments except the 2nd and 3rd thoracic and the two last abdominal. In Apis there are eleven pairs of tracheal invaginations according to Kowalevsky (No. 416), but according to Bütschli (No. [405]) only ten, the prothorax being without one. In the Bee they appear simultaneously, and before the appendages.
The blind ends of the tracheal invaginations frequently (e.g. Apis) unite together into a common longitudinal canal, which forms a longitudinal tracheal stem. In other cases (e.g. Gryllotalpa, Dohrn, No. [408]) they remain distinct, and each tracheal stem has a system of branches of its own.
The development of the tracheæ strongly supports the view, arrived at by Moseley from his investigations on Peripatus, that they are modifications of cutaneous glands.
The salivary and spinning glands are epiblastic structures, which in their mode of development are very similar to the tracheæ, and perhaps have a similar origin. The salivary glands arise as paired epiblastic invaginations, not, as might be expected, of the Stomodæum, but of the ventral plate behind the mouth on the inner side of the mandibles. At first independent, they eventually unite in a common duct, which falls into the mouth. The spinning glands arise on the inner side of the second pair of maxillæ in Apis and Lepidoptera, and form elongated glands extending through nearly the whole length of the body. They are very similar in their structure and development to salivary glands, and are only employed during larval life. They no doubt resemble the mucous glands of the oral papillæ of Peripatus, with which they have been compared by Moseley. The mucous glands of Peripatus may perhaps be the homologous organs of the first pair of maxillæ, for the existence of which there appears to be some evidence amongst Insects.
Mesoblast. It has been stated that the mesoblast becomes divided in the region of the body into two lateral bands ([fig. 189] A). These bands in many, if not all forms, become divided into a series of somites corresponding with the segments of the body. In each of them a cavity appears—the commencing perivisceral cavity—which divides them into a somatic plate in contact with the epiblast, and a splanchnic plate in contact with the hypoblast ([fig. 189]). In the interspaces between the segments the mesoblast is continuous across the median ventral line. The mesoblast is prolonged into each of the appendages as these are formed, and in the appendages there is present a central cavity. By Metschnikoff these cavities are stated to be continuous, as in Myriapods and Arachnida, with those of the somites; but by Hatschek (No. [414]) they are stated to be independent of those in the somites and to be open to the yolk.
The further details of the history of the mesoblast are very imperfectly known, and the fullest account we have is that by Dohrn (No. [408]) for Gryllotalpa. It would appear that the mesoblast grows round and encloses the dorsal side of the yolk earlier than the epiblast. In Gryllotalpa it forms a pulsating membrane. As the epiblast extends dorsalwards the median dorsal part of the membrane is constricted off as a tube which forms the heart. At the same time the free space between the pulsating membrane and the yolk is obliterated, but transverse passages are left at the lines between the somites, through which the blood passes from the ventral part of the body to corresponding openings in the wall of the heart. The greater part of the membrane gives rise to the muscles of the trunk.
Ventrally the mesoblastic bands soon meet across the median line. The cavities in the appendages become obliterated and their mesoblastic walls form the muscles, etc. The cavities in the separate mesoblastic somites also cease to be distinctly circumscribed.
The splanchnic mesoblast follows the hypoblast in its growth, and gives rise to the connective tissue and muscular parts of the walls of the alimentary tract. The mesoblastic wall of the proctodæum is probably formed independently of the mesoblastic somites. In the head the mesoblast is stated to form at first a median ventral mass, which does not pass into the procephalic lobe; though it assists in forming both the antennæ and upper lip.
The alimentary canal. The alimentary tract of Insects is formed of three distinct sections ([fig. 181])—a mesenteron or middle section (me), a stomodæum (st) and a proctodæum (an). The stomodæum and proctodæum are invaginations of the epiblast, while the mesenteron is lined by the hypoblast. The distinction between the three is usually well marked in the adult by the epiblastic derivatives being lined by chitin. The stomodæum consists of mouth, œsophagus, crop, and proventriculus or gizzard, when such are present. The mesenteron includes the stomach, and is sometimes (Orthoptera, etc.) provided at its front end with pyloric diverticula—posteriorly it terminates just in front of the Malpighian bodies. These latter fall into the proctodæum, which includes the whole of the region from their insertion to the anus.