The procephalic lobes.—The head-lobes, or procephalic lobes, appear at a very early period (Fig. 524, kl), before any traces of the segments of the trunk region. Ayers has shown that in Œcanthus the primitive band, in its earliest condition and before the appearance of the head-lobes, is a simple oval plate or almond-shaped thickening near the posterior end of the egg (Fig. 525, 1, 2). This plate is “soon divided into two tolerably well-marked regions by the enlargement of the head-end,” the first indication of the head-lobes (3). A depression next forms in what is to be the middle of the forehead. “It indicates the position of the future labrum, and forms the inner boundaries of the two cephalic ganglia, which are developed on either side of this depression at a much later stage.” Almost simultaneously with the appearance of this depression, two lateral folds are formed in the trunk portion of the band, which are the first indications of the maxillary and thoracic regions, the abdominal portion not yet showing traces of a division into segments (Fig. 525, 5). The thickened outer edges of the head-fold next gradually grow in towards the median line (Fig. 525, 5), and bend forward towards the region of the future mouth. The rounded angle made by the posterior end of the head-fold is the first indication of the antennæ. The embryo is now composed of four well-marked regions: cephalic, maxillary, thoracic, and abdominal. The primitive band then grows much longer, the primitive mouth and anus appear, and the appendages bud out, and eventually the embryo revolves and appears on the ventral side of the egg (Fig. 525, 6).
Fig. 525.—Early stages in the embryology of Œcanthus niveus. Fig. 1, the youngest observed primitive band, the serosa not yet formed; 2, longitudinal optic section (diagrammatic) of Fig. 1; 3, the primitive band after the appearance of the head-fold, which is indicated at this time by the more rapid growth and consequent greater breadth of the lower end of embryo, x 25; 4, a young embryo after the appearance of the primitive segment-folds, x 50; 5, a more advanced embryo, with the antennal folds distinctly marked off; the free ends of the primitive folds have united across the embryo posterior to the antennal folds, x 50; 6, ventral view of the embryo with the appendages budding out, x 25 (the embryo in this stage lies dormant through the six colder months of the year): am, amnion; m, micropylar end; ch, chorion; gb, primitive band; bf, brain-fold; yl, yolk; tf, caudal fold; kf, head-fold (procephalic lobe); p.fd.t, primitive thoracic fold; p.fd.m, primitive maxillary fold; p.fd.a, primitive abdominal fold; ab.c, abdominal constriction; t.c, thoracic constriction; at.l, antennal lobe; M, mesoderm; h.g, head groove; mo, mouth; sk invagination of ectoderm to form head-apodeme; md, rudiment of mandible; m1, 1st, m2, 2d maxilla; T1–T5, legs: ab.p, 1st abdominal appendage; ap, other appendages; tb, caudal expansion; mf median furrow; B, primitive unpaired organ (metastomum).—After Ayers.
These primitive regions of the primitive band, before the segments are formed, are called by Graber macrosomites, and the secondary segments into which they divide (which afterwards become the body-segments), microsomites. The macrosomites are peculiar to insects, and may be an inheritance from a hypothetical ancestral form. With Korschelt and Heider, we should hardly share this view.
Fig. 526.—Older embryo of Œcanthus with the appendages budded out, those of the abdomen distinct: abp, first pair; a.s, anal stylet; pr, proctodæum; am, amnion.—After Ayers.
Our observations on locusts show clearly (1) that the procephalic lobes are the pleural portion of the first cephalic or antennal segment; (2) that the antenna is an appendage or outgrowth of the procephalic lobes; (3) that the eyes are a specialized group of epidermal cells of the upper part of the procephalic lobes, and are not homologues of the antennæ or of the appendages in general; and (4) it seems to follow from a study of the relations and mode of development of the clypeus and labrum, that they arise between the procephalic lobes, and probably represent the tergal part of the antennal segment, forming the roof of the mouth, i.e. closing in from above the pharynx.
In general the formation of the body-segments into the primitive band is in succession from before to the hinder end. This successive appearance has been observed by Graber in genera of different orders (Stenobothrus, Lina, and Hylotoma). For example, in the beetle Lina, after the appearance of the mandibular and two maxillary segments, appear the three thoracic segments, together with the two anterior abdominal segments, the other abdominal segments arising afterwards. In other cases, the formation of segments seems to be simultaneous along the entire length of the primitive band. An exception to the rule has been noticed by Heider in Hydrophilus, as in this beetle the development of the segments of the middle region appears somewhat delayed, while the fore and hind parts of the primitive band are more rapid in development. In Pieris, according to Graber, the thoracic segments are more rapidly developed than the others; soon after, the gnathic segments (mandibles and two pairs of maxillæ) appear, and finally the abdominal segments are formed.
Fore-intestine (stomodæum) and hind-intestine (proctodæum), Labrum.—The digestive canal of insects consists, as in other animals, of three portions, the fore, mid, and hind gut or intestine. The next change after the completion of the segments of the primitive band is the development of the fore and hind intestine and the appendages. The fore-intestine (stomodæum) arises as an invagination in the area of the primary head-section, and the hind-intestine (proctodæum) in the terminal section (Figs. 300 and 546).
In insects generally the formation of the fore-intestine occurs earlier than that of the hind-intestine. An exception was discovered by Graber and also by Voeltzkow in Muscidæ, where the proctodæum appears earlier.