On the other hand the view of Haase, that the prolegs of caterpillars are secondary, adaptive characters, is supported by the fact of the rapidity with which two pairs on the 3d and 4th segments nearly disappear in the larvæ of certain Noctuidæ (Catocala, etc.), a reduction evidently due to disuse.
The tracheæ.—The tracheal system arises as ectodermal invaginations on one side of the appendages, appearing soon after the latter. The earliest condition of the tracheal invagination is seen in section at Fig. 539, E, tr; as it deepens, it sends off diverticula or tracheal branches, while the narrow mouth of the invagination forms the stigma. The cup-like cavities situated serially one behind the other, and arising from the single tracheal invaginations, become at the end or bottom of the cup elongated along the length of the body and fused together at their ends; then the two longitudinal stems of the system arise, by a breaking through at the place where the original invagination had become fused, thus forming a continuous tube, the lumina opening into each other. (Bütschli.)
The cuticular tracheal intima is differentiated late in embryonic life. The entrance of the air is accomplished in part before the embryo hatches, the air being derived from the tissues and fluids of the body.
The farther development of the tracheal branches is due to the progressive formation of diverticula. The branches thus arising are intercellular formations. On the other hand, the finest twigs are intercellular structures. However, as Schaeffer states, the differences between the two modes of formation are not important.
Wheeler mentions the existence of “two pairs of very indistinct tracheal openings in the 10th and 11th somites” of the abdomen of Doryphora (Fig. 546, t19, t20), and Heider believes that they exist in Hydrophilus.
The tracheal invaginations as a rule begin to appear after the appendages commence to bud out. An exception is met with in the bee (Apis), where the tracheal ingrowths are seen before the rudiments of the legs. Most of the tracheal invaginations appear simultaneously. Only rarely do we see an indication of their successive development from before backwards. Thus in Hydrophilus, Graber observed that the mesothoracic stigmata appeared somewhat earlier than those of the other segments.
h. Nervous system
The rudiments of the nervous and tracheal systems essentially contribute to the building up of the relief of the primitive band of insects. The nervous system is the earliest to appear, being indicated very early, in fact before the appendages begin to grow out. The first traces of the nervous system are two ridges extending along the primitive band, the depression between them being called the primitive furrow. At an early period the segmentation is observed in the primitive ridges, while widened spaces (the rudiments of the ventral ganglia) alternate segmentally with the narrow places which are the incipient longitudinal commissures (Fig. 527, A, g).
The primitive ridges extend anteriorly into the head-lobes; this part must be regarded as the rudiment of the œsophageal commissure. The rudiments of the brain are from their first appearance directly connected with the ventral chain of ganglia.[[83]]
Completion of the definite form of the body.—This is accomplished by the growth of the primitive band around the yolk, the band widening, so that its edges behind the head extend up, and finally meet on the back, forming the back or tergum of the embryo, thus enclosing the yolk (Fig. 530, F). The tergal wall of the head is due to the dorsal growth of the head-lobes, and of the clypeo-labral region. In the course of this process the anterior end of the primitive band becomes turned up dorsally, forming a dorsal curve or bend. By this bending up of the primitive band the forehead nearest the mouth forms a transverse ridge, the labrum, while the basal or earlier part of the forehead now is differentiated into the clypeus. This clypeo-labral region likewise forms the roof or palatal region of the mouth. The head-lobes cause by this dorsal growth a rotating motion which carries the rudimental antennæ back over the mouth.