-» VIII * THE DIPLOPODA THE diplopods are distinguished from all other arthropods by the fact that most of their body segments are "double" and, as implied in the group name, carry each two pairs of legs. The double nature of the segments is evident not only from their two pairs of legs but also in the presence of two pairs of ganglionic centers in the ventral nerve cord, two pairs of lateral pouches of the male gonads, and two pairs of cardiac ostia and alary muscles. The double segments are termed diplosomites. Since it would seem most probable that the diplopods have been derived from ancestors having single segments, it may be supposed that the diplosomites represent two primitive segments united; in their ontogenetic development, as shown by PHugfelder (1932), the double segments are formed from two con­ secutive sets of somite rudiments that are not differentiated into separate segments. It is not clear what advantage the diplopods have gained from having two pairs of legs on each functional seg­ ment, or, otherwise stated, in having the segments united in pairs. By comparison with the agile chilopods, they are sluggish creatures, their gait is a leisurely crawl, and they make no effort to escape capture. Their curious organization, however, gives the diplopods a distinctive place among the arthropods. In certain features of the head and the mandibles the diplopods are evidently related to the chilopods, while, on the other hand, there is little doubt that they are allied to the pauropods and the symphylids. The four groups, therefore, have generally been classed together as the Myriapoda. The Diplopoda, Pauropoda, and Sym­ phyla, however, differ from the Chilopoda in that the genital open- 225 ARTHROPOD ANATOMY ings are near the anterior end of the body, instead of at the posterior end. They are hence distinguished as the Progoneata, or progoneate myriapods, in contrast to the opisthogoneate chilopods. It is possible, however, that the progoneate condition is not primary, since Tiegs ( 1940 ) has shown that there is evidence in the ontogenetic develop­ ment of the symphylids of a primitive posterior connection of the genital ducts with the exterior. The definitive anterior exit ducts, he says (1947 ), in both Pauropoda and Symphyla are not persisting coelomoducts but ectodermal ingrowths. The class Diplopoda includes a large number of species, which taxonomists divide into three principal groups : the Pselaphognatha, the Opisthandria, and the Proterandria. The Pselaphognatha are mi­ nute forms comprised in the family Polyxenidae, having not more than 13 body segments, and are recognizable by the presence of large tufts of hair on the back and sides of the body. The Opisthandria are of larger size, and the males are characterized by having the last pair or the last two pairs of legs modified as clasping organs by which the male in copulation holds the female, while he places the sperm in her genital openings by means of his mandibles. The Proterandria include most of the more common types of diplopods; their distin­ guishing feature is the modification in the male of one pair or both pairs of legs, usually on the seventh body segment, to serve as the functional intromittent organs, termed gonopods, by which the sperm is transferred from the male genital openings on the third body segment to the receptacles of the female on the corresponding seg­ ment. The Proterandria are divided into four orders : the Polydes­ moidea, the Nematophora, the Juliformia, and the Colobognatha. Representatives of the Polydesmoidea and the Juliformia will be described here as examples of the more familiar types of diplopods. For a full account of the structure and classification of the diplopods the student is referred to the comprehensive works of Silvestri (1903 ), Verhoeff (1926-1931 ), and Attems (1926b). The diplopods feed principally on decaying vegetable matter, particularly on the dead leaves of certain kinds of trees and shrubs; fresh leaves and fruit they seldom eat, and carnivorous habits have been observed only in members of one family, which are said to at­ tack earthworms and phalangids. The alimentary canal is a wide, simple tube extending straight through the body, and has a single pair of excretory Malpighian vessels arising from the anterior end 226 THE DIPLOPODA of the proctodaeum. Salivary glands of the anterior part of the head open into the preoral cavity through the epipharyngeal wall, and glands in the anterior region of the body discharge on the base of the inner wall of the gnathochilarium. A distinctive feature of the diplopods is the nature of the respira­ tory system. The breathing organs are bundles of tracheae given off from hollow sternal apodemes that are open to the exterior by apertures called spiracles, closely associated with the bases of the legs. Descriptions of the apodemes and tracheae of various diplo­ pod genera will be found in papers by Effenberger (1907), Ziegler ( 1907), Krug (1907), Wernitzsch (1910), Reinecke (1910), Voges ( 1916), Ripper (1931 ), and Seifert (1932). In general, the tracheae are fine, unbranched tubes, with spiral taenidia, and do not unite with each other, but in the Polyxenidae and Glomeridae they are branched. The apodemes, which are variously developed, serve also for the attachment of the leg muscles, and usually the apodemes of the first three body segments have no tracheae. It seems probable that the apodemes were developed originally for muscle attachments; the spiracles of the diplopods evidently can have no relation to the lateral spiracles of the chilopods and insects. In their mode of development the diplopods are anamorphic, as are the Scutigeromorpha and Lithobiomorpha among the chilopods. The young of most species leave the egg with only three pairs of legs, which are the first three pairs of the adult, and have not more than seven body segments. The definitive segmentation is acquired gradually at the subsequent moults, accompanied by an increase in the number of legs. The new segments are formed in a zone of growth at the posterior end of the body, which remains active throughout life. Inasmuch as adult diplopods of different groups differ much in the number of body segments, the anamorphic pro­ gression of segment formation is not the same in all of them. A de­ tailed account of the manner of segment generation in the zone of growth is given by PHugfelder (1932). The postembryonic develop­ ment of lulus is described by Krug (1907), that of several species of Polydesmoidea by Miley (1927), PHugfelder (1932), and Seifert ( 1932), and a general account of diplopod anamorphosis is given by Attems (1926b). Each successive stage of growth is accompanied by a moult, preliminary to which the young diplopod encloses itself in an earthen nest. At each moult the head capsule of the exuviae is 227 ARTHROPOD ANATOMY disarticulated from the body, and ecdysis is accomplished by the animal's crawling out of the old cuticle of the trunk. Miley (1927) observes that in Euryurus the shed cuticle is "split down the mid­ ventral line and also somewhat laterally, just above the articulation of the legs on one side or on both sides." The Head and the Mouth Parts The diplopod head (fig. 63 ) is strongly convex above (D) and Battened beneath. Eyes are present in some forms (C, D) and ab­ sent in others (A). The antennae arise well back from the anterior margin, so that before them is a wide epistomal area (A, D, Epst ) , but the latter is in no way specifically separated from the rest of the cranial wall. With the margin of the epistome is united the nar­ row, dentate labrum (A, C, Lm). On the sides of the head are the large, convex bases of the mandibles (A, B, C, D, mdB ), and the ventral surface of the head is covered by a broad Hat appendage known as the gnathochilarium (B, D, Gnch). The antennae are seven-segmented appendages (A, Ant), as are the legs, and each segment is individually musculated. The eyes, when present, have superficially the appearance of compound eyes (C, D), but they are merely groups of simple eyes like those of the chilopods, as seen at F, C, and H on figure 69. Postantennal organs of Tomosvary are present in most of the diplopods, except Juliformia and Colobognatha. In some forms the sensory cells lie beneath open pits or grooves; in others they are en­ tirely covered by the cuticle. In the polydesmoids each organ is marked externally by an oval groove of the cuticle with a thickened central disc (fig. 63 A, OT); the sense cells lie beneath the groove and have no communication with the exterior. Beneath the epistome (fig. 63 F, Epst) is a long palatal, or epipharyngeal, surface (Ephy ) which extends posteriorly from the labrum (Lm) to the mouth (Mth ) and forms the roof of a large preoral food cavity (Pre ) closed ventrally by the gnathochilarium. At the base of the inner surface of the gnathochilarium is a group of Battened suboral lobes that constitute the hypopharynx (Hphy ) , which in the diplopods does not have the form of a projecting tongue­ like organ, such as that of the chilopods and insects. The preoral cavity is occupied mostly by the gnathal lobes of the mandibles, which close against the hypopharynx. The epipharyngeal surface 228 THE DIPLOPODA usually presents a transverse ridge behind the labrum (E) and a low median elevation. The segmental composition of the diplopod head is a subject on �r, m l ' I' /.� Ephy c�_/ E Poe D �--,r-_ Mth meR/ E pI;"y \ Ep�t " Lm-_ Fig. 63. Diplopoda-Polydesmoidea and Juliformia. The head. A, Apheloria coriacea (Koch ), head, anterior. B, Habrostrepus sp., head, ven­ tral.