The Evolution of Arthropodan Locomotory Mechanisms. Part 71. Functional Requirements and Body Design in Colobognatha (Diplopoda), Together With a Comparative Account of Diplopod Burrowing Techniques, Trunk Musculature and Segmentation

A record is given of some habits and capabilities of species belonging to three of the four superfamilies of Colobognatha which appear to be associated with the morphological evolution of the order (p. 389). 2. A method of ‘wedge’ burrowing employed by Colobognatha culminates in the ability of Dolistenus to exert a pushing force relative to its size some three times greater than that of any other diplopod which has been recorded (p. 390). 3. The six species of Colobognatha which have been obtained alive appear to be particularly resistant to desiccation, culminating in Siphonotus whose preference for dryness is remarkable while damp conditions are lethal to it (p. 386). 4. Lateral flexibility is conspicuous in Siphonophora, permitting horizontal enrolment on a ceiling (p. 389). 5. In Siphonophora the phase relationship between legs of a pair is variable, as in Onychophora and Geophilomorpha. It is suggested that where legs are short and gaits are medium in pattern, the longitudinal co-ordination between successive legs on either side of the body is, for mechanical reasons, more important than the transverse co-ordination (p. 391). 6. The morphological bases of 2 and 4 have been assessed as a result of a comparative survey of diplopod skeleto-musculature and achievements. 7. A brief comparison is given of the three principal diplopodan burrowing techniques:— head-on pushing; dorsal surface pushing; and wedge pushing. Intemediates exist between the three extremes (p. 392). 8. A Functional and comparative account of colobognathan skeleton is given which includes several types of sternite within the same order (p. 395). 9. Siphonotus and Polyzonium possess purely respiratory thin-walled tracheal sacs, the trunk and limb muscles inserting on to the edges of the sternites and on to special ‘sternite apodemes’ not present in Dolistenus or in the Eugnatha (p. 401). 10. Siphonophora shows a partial union between respiratory tracheal pouches and sternite apodemes. The condition of these series of structures in Siphonotus, Polyzonium, Siphonophora and in typical Eugnatha suggests a possible manner of evolution of skeletal and respiratory tracheal pouch apodemes which are typical of the class Diplopoda (pp. 402 and 455). 11. A functional and comparative survey is given of trunk musculature found in the principal diplopod orders. The muscles fall almost entirely into a common scheme, modified greatly according to needs in the various orders and species (p. 405 and text-fig. 35). 12. The structure, musculature and uses of colobognathan legs are described (p. 426). 13. The musculature of typical trunk segments in Dolistenus is more specialized than that of other diplopods which have been examined, and, with its skeletal features, accounts for the large pushing force exerted by this animal (p. 428). 14. The ventral musculature of Polyzonium, Siphonophora and Siphonotus differs from that of all other diplopods in its insertions. Siphonophora shows intersegmental attachments of part of the ventral longitudinal muscles, such as found in Pauropoda and Symphyla (p. 455). 15. The musculature of the head end of the body of selected diplopods is described in relation to function and probable phylogeny. Specializations of muscles in the anterior few segments facilitating enrolment occur in somewhat similar manner, but from different sets of muscles in different animals:—from the superficial obliques in Cylindroiulus, from the deep obliques in Polymicrodon (p. 429), and from the pleuro-sternals in Dolistenus (p. 436). A retractor obliquus dorsalis appears to have been evolved independently in the Oniscomorpha and Siphonophora (pp. 415 and 437). This diversity suggests that the ability to spiral evolved in parallel manner within the Diplopoda, and after the ability to push strongly by diplosegments had been evolved (p. 444). 16. The anterior muscular specializations facilitating wedge burrowing in Dolistenus are unique, and different specializations serve the needs of the probing proboscis in Siphonophora and Siphonotus (pp. 436 and 437). 17. Muscles of the collum segment can be related to those of a typical trunk segment, but they are modified in relation to the absence of walking legs and to the various needs of the nodding movement in the several orders (p. 439). 18. The presence of a typical legless sternite 1 on the collum segment of Polyxenus is confirmed (p. 449). 19. New evidence concerning the serial homologies of the parts of the anterior trunk segments of a number of diplopods is presented. There is functional significance in the forward shift of sternites relative to tergites which occurs to various extents in different diplopods. No shift occurs in the opposite direction (pp. 398 and 448). 20. The suggestion that the first leg pair of Polyxenus belongs to the collum segment is shown to be without foundation, the anterior segmentation of this animal being similar to those of other diplopods. The scarcity of primitive diplopodan features in Polyxenus is further confirmed (pp. 451 and 453). 21. The interpretation put forward by the older systematists concerning the anteriorly directed sternal shifts and fusions in segments 4 and 5 of the Spiroboloidea is substantiated, and the newer interpretation put forward by Snodgrass is shown to be without foundation (p. 444). 22. Evidence is presented which demonstrates that trunk segment 4 in the Colobognatha carries but one leg pair and does not differ in condition from that of other diplopods (p. 448). 23. Evidence from the muscles suggests that segments 2-4 are similar in composition. Least assumptions are required for the supposition that these segments are diplosegments in which one sternite and leg pair have been lost. Deep oblique and pleuro-sternal muscles, characteristically serving two ventral units, are concentrated on to the single ventral unit of these segments in some diplopods (p. 445). 24. The supposed primitiveness of the Colobognatha is discussed in the light of the above findings. It is shown that in many respects the Colobognatha are structurally and functionally as specialized as any other order, and in directions which do not lead towards the structure and capabilities of other diplopods. A few colobognathan features appear to be primitive, but some features of this order appear in a more primitive condition in eugnathan orders (p. 454). 25. A wider range of form of body scutes, head and musculature exists among Golobognatha than in any other order of diplopods. Diagnostic features of Golobognatha become fewer with advancing knowledge of colobognathan structure. 26. Since this is the fullest comparative survey of the musculature of the major orders of any single class of arthropods, the changes undergone by muscles which have apparently shifted one or both insertions are listed (p. 457). The functional significance of these changes are known.