The Post-embryonic Development of Hanseniella agilis (Symphyla)
Authors: Tiegs O. W. 
Published: April 2021

ABSTRACT
In the newly emerged larva there are seven leg-bearing segments. Behind these are two apparent segments, viz. the ‘pre-anal piece’ and the ‘anal piece’, the former bearing the cerci and developing eighth legs, while the latter bears the trichobothria. These apparent segments are not identical with the true anal and pre-anal segments, the segmentation at the posterior end of the abdomen being obscured by the new segments which are in process of formation there. The clue to the segmentation is furnished, not by grooves in the external chitin, but by internal organs, such as the ‘ventral organs’, ganglia, and muscles, as well as by the appendages. The ‘anal piece’ comprises the true anal segment, as well as the sternal wall of the pre-anal segment; the tergal wall of the ‘pre-anal piece’ belongs to the pre-anal segment, but its sternal wall is the sternal wall of the developing eighth abdominal segment. At the first ecdysis part of the tergal wall, and the whole of the sternal wall of the ‘pre-anal piece ‘, together with its developing appendage move forward to form the new eighth segment, while the ‘preanal piece ‘becomes reconstituted by incorporation and enlargement of a new sternal wall, which, together with the developing ninth leg, has become constricted off from the ‘anal piece’. The mature ninth segment, together with the second tergal scute of the eighth segment, appear at the second ecdysis ; the tenth and. eleventh at the third and fourth ecdyses. The fifth ecdysis reveals the ‘adult’ twelve-legged animal; the new segment, however, remains part of the ‘pre-anal piece ‘, thereby preserving the misleading picture of the terminal segmentation. (In the genus Symphylella, however, the twelfth segment becomes demarcated from the pre-anal.) Growth, attended by periodic ecdyses, continues after the ‘adult’ condition is attained. The epidermis not only of the pre-anal, but probably also of the anal segment furnishes the material from which the wall of the new segments develops. The antenna of the newly emerged larva is six-segmented. Formation of new segments out of the basal segment attends the successive ecdyses, even after the adult twelve-legged condition has been reached. In the second instar larva a pair of accessory cerci (‘pygidial cones’) are present below the main cerci; after the second ecdysis they become the functional cerci, the original cerci undergoing resorption. The second cerci become replaced in a similar manner after the fourth ecdysis by the third pair of cerci, and these survive as the permanent cerci. With each new cercus there forms a new spinning gland, the effete gland undergoing degeneration. Enlargement of the general epidermis of the larva is attended by proliferation of its cells, which decrease markedly in size. The tracheal system does not become functional till after the first ecdysis. Its growth is attended by much elaboration of its system of branches, and by the eventual fusion of the two main tracheal trunks under the pharynx. There is no evidence of division amongst the cells of the tracheal epithelium. In the pre-anal segment the ‘ventral organs ‘of the embryo survive into the larva. They are loci of active cell-division, and provide the material for the formation of new ganglia and eversible sacs. The embryonic development of these organs is therefore repeated in the larva. The new ganglia arise by proliferation of cells from the inner zone of the ‘ventral organs ‘, and move forwards with the new segments. The eversible sacs become constricted off from the inferior part of the ‘ventral organs ‘as the new segments become demarcated.. After the formation of the twelfth ganglion the fused anal and pre-anal ganglia merge incompletely with it, and so come to lie within the floor of the twelfth segment. The mesodermal organs of the teloblastic segments develop out of the already formed mesoderm of the growing zone, both the anal and pre-anal coelomic sacs being involved. In the eight-day embryo the anterior ends of the two pre-anal coelomic sacs co-operate with those before them in the enclosure of the mid-gut, and in the formation of the heart. From each pre-anal coelomic sac there is thus formed, as in the preceding coelomic sacs, a membranous partition, in which the coelomic cavity is almost obliterated. This membranous septum is associated above with the developing heart, while behind it gradually merges with a similar septum from the anal somite. In anticipation of its future role in the formation of the pericardial septum, a thin but continuous membrane, with partially obliterated coelomic cavity, is thus present at the time of eclosion, connecting the hinder end of the heart with the entire mesoderm along the growing zone. This mesoderm comprises, in addition to anal and pre-anal coelomic sacs: (i) the eighth coelomic sac, which develops from the floor of the pre-anal somite, even before the eighth leg has appeared; (ii) the ‘unsegmented mesoderm’ which extends back from the pre-anal coelomic sac, along the floor of the abdomen, to the anal somite. The eighth coelomic sac disrupts, in the usual way, into mesenchymatous cells, out of which the musculature of the eighth segment and its appendage develops. The ‘unsegmented mesoderm’ then enlarges, partly by proliferation of its own cells, and partly by incorporation of cells from the anal somite, and, with the formation of the ninth leg near the time of eclosion, gives rise to the ninth somite. The tenth, eleventh, and twelfth somites form in a similar manner early in the second, third, and fourth larval stadia, and, like the ninth, develop conspicuous coelomic cavities. The further development of all these coelomic sacs proceeds as for the eighth, already described. The anal coelomic sac is not recognizable beyond the fourth larval stadium. The pericardial septum of each new segment forms out of the membranous partition already described, its most anterior part separating off, and moving forwards with each successive new segment. At its upper end the membrane contributes cardioblasts to extend the heart into the new segments. A vestige of the coelomic cavity survives at its side as sinus lateralis cordis. The genital tube, like the heart, enlarges behind by incorporation of cells from each developing pericardial septum, whose definitive condition is attained by the separation of the genital tube from it. After the last larval moult the genital tubes become detached behind and drawn forward a little in the haemocoele. In the female the entire genital tube of the larva survives as ovarian tube in the adult ; the genital atrium and oviducts form as epidermal ingrowths in the nine- or ten-legged larva. The genital atrium of the male and the ejaculatory ducts are also epidermal. In the male only the hinder part of each genital tube will form the wall of the testis ; out of its anterior part are developed the vas deferens and vesicula seminalis. From the primordial germ-cells of the developing testis are formed: (i) spermatogonia, which themselves undergo further multiplication ; (ii) additional primordial germ-cells, out of which a continuous dorsal germarium is formed along each testis. The interior of the enlarging testis becomes divided into irregular compartments by ingrowths from the wall of the testis; within the several compartments the spermatogenesis is remarkably synchronized. Compartments with mature sperms appear at random along the testis, the discharge of such sperms requiring their passage through the whole length of the compact organ. In the wall of the vesicula seminalis the sperms may exhibit a peculiar intracellular phase, of unknown meaning. Precocious sexual maturity occurs in some male eleven-legged larvae; in others the discharge of sperms from the testis is long delayed and then the testis may become extremely large. In the female the primordial germ-cells multiply and clump together to form the germarium, within which the preliminary phases of meiosis take place. In the nine-legged larva a second germarium appears. From the germaria oocytes are discharged into the lumen of the ovary, and, having become enclosed each in a capsule of cells derived from the wall of the ovary, gradually enlarge. After an interval of months yolk begins to accumulate in the cytoplasm. Reduction division of the nucleus takes place at about the time of laying. Some females show sexual precocity. The progoneate condition of the Symphyla appears to be a secondary adaptation to anamorphosis ; it does not seem to be of sufficient importance to outweigh all the other evidence pointing to a close affinity of Symphyla with the immediate ancestors of insects.. In the growing larva the already formed muscle-fibres enlarge by growth of their contractile substance; there is no formation of new muscles in the old segments. The developing muscles of the new segments are a useful index for defining the limits of these segments while they are still part of the ‘preanal piece ‘. The already formed parts of the heart and blood-vessels grow by cell-enlargement. The heart-wall of the new segments is formed out of cardioblasts furnished by the developing pericardial septum of the growing zone. The mesoderm of the growing zone contributes cells for the posterior elongation of the ventral blood-vessel. The phagocytic and nephrocytic tissues have been identified by injection experiments: The phagocytic tissues are: (i) a pair of anal phagocytic organs into which the terminal bifurcation of the ventral bloodvessel empties, (ii) the pericardial tissue and free leucocytes, (iii) the ‘funnel ‘of the aorta, (iv) the pericardial septum, (v) the neurilemma. The nephrocytic tissues are: (i) the paracardial nephrocytic organs, (ii) the salivary gland ‘end-sac’. The anal phagocytic organs first appear in the eleven-legged larva. In the paracardial nephrocytic organs growth is attended by much cell-division. The fat-body is derived from yolk-cells that have surv’ ved from the embryo. In the first instar larva the remaining yolk becomes absorbed. The irregular reticulum of protoplasm now becomes more compact and cell-walls appear within it. The nuclei multiply by mitosis. As the larva grows the usual storage products form in the fat-body. Massive accumulations of excretory concretions also form in the fat-body, but there are no specialized urate cells. The spinning glands of the first instar larva are replaced in the third instar by a second pair of spinning glands, which develop in association with the first accessory (second functional) cerci. These glands again become replaced in the fifth instar larva by the third (and final) pair of spinning glands, the latter developing in association with the second accessory (final) cerci. Although there is considerable enlargement of the foregut and hind-gut cells in the growing larva, there is also evidence of periodic cell-division. In the fore-gut this takes place in a special growing zone at its hinder end ; but in the hind-gut, with its structurally distinct regions, mitoses appear at random throughout its length. The mid-gut epithelium is subject to periodic renovation. There is one, and probably only one, such renovation for each larval instar, but the processes of renovation continue in the adult animal. Regeneration of the epithelium does not take place from local ‘replacing cells’, but from a specialized regeneration zone at its hinder end. The new epithelium, advancing from the hinder end, strips off the old epithelium, which then slowly undergoes disruption in the mid-gut cavity enclosed by the new epithelium. In the Malpighian tubes mitoses appear at random along their length, but are specially abundant in their thickened basal ends.