Rhachodesmidae

Medial (left) and lateral views of gonopods of Neoleptodesmus geniculatus, from Kraus (1954).

Belongs within: Polydesmida.

The Rhachodesmidae are a family of diverse and often brightly-coloured millipedes found in Mexico and Central America.

Stunning Central American millipedes
Published 23 September 2014
Blue cloud forest millipede Pararhachistes potosinus, copyright Luis Stevens.

For my semi-random selection of taxon to write about this week, I drew the millipede family Rhachodesmidae. Rhachodesmids are members of the millipede group called the Polydesmida, characterised by the presence of lateral keels on each segment of the body. The presence of these keels had lead to platydesmidans sometimes being referred to as ‘flat-backed millipedes’ though depending on how strong the keels are, not all species are necessarily ‘flat-backed’.

In an earlier post on millipedes, I stressed the importance of genitalia in characterising millipedes, and the Rhachodesmidae are no exception. In polydesmidans, it is the front pair of legs on the seventh segment that is modified into the gonopods in males (with one notable exception that I may refer to later). Gonopods of rhachodesmids lack the solenite or coxal spur found in many other polydesmidans, and the inner side of the gonopod has a distinct elongate or oval concavity that is densely setose. Other noteworthy features of rhachodesmids are that they are often relatively large, with a conical terminal segment and more or less thickened rims to the lateral keels (Loomis 1964).

An unidentified rhachodesmid, copyright Sergio Niebla.

Beyond that, rhachodesmids become a little more difficult to characterise. Though they are not a widespread group, being restricted to Mexico and Central America, they are very diverse in appearance. Loomis & Hoffman (1962) commented that, “Rhachodesmoids collectively are members of a group notable for great variability and the development of bizarre features. Among their ranks we find millipeds which are bright blue, green, orange, and even pure white as adults; here the gonopod structure ranges from the normal polydesmoid appearance down to monoarticular fused remnants. Body form varies from a slender juliform shape to broad, flat, limaciform contour. Within the limits of this so-called single family occurs more variation than in all of the remaining polydesmoids.” They also noted that the group was in need of review, something that apparently remains undone to this day (though there is someone working on it). If the photographs I’ve commandeered in this post are any indication, this is definitely a group that deserves more love.

Paratype of Tridontomus procerus, from Loomis & Hoffman (1962).

Loomis & Hoffman (1962) made their comments in comparing the Rhachodesmidae to another Central American polydesmidan family they were then describing as new, the Tridontomidae, and if I’m referring to the rhachodesmids then I should probably give a shout-out to these remarkable beasts as well. So far as I’ve found, this family is still only known from two species, Tridontomus procerus and Aenigmopus alatus, from Guatemala. Not only is the appearance of tridontomids striking, with long spinose processes on either side of the body, but the genital morphology of one species, A. alatus, is especially bizarre: it doesn’t have any where it should. Where males of other polydesmidans have the legs of the seventh segment modified into gonopods, those of A. alatus have a perfectly ordinary pair of walking legs. In normal polydesmidans, the gonopods are used to transfer sperm from seminal processes on the coxae of the second pair of legs to the female’s genital opening (more details are available here), but obviously Aenigmopus must do things differently. The seminal processes are still present, and the second legs themselves are thickened compared to other millipedes; it is possible that they are somehow used to transfer sperm directly from process to female without the use of gonopods. However it does it, there is no question that Aenigmopus is unique in the world of polydesmidans.

Systematics of Rhachodesmidae

Characters (from Loomis 1964): Body incapable of being tightly rolled; segments 20; segment 1 narrower than head; pores opening from thickened outer rim of keels; gonopods with definite elongate or oval cavity in mesal face containing many setae; last segment conical at apex.

<==Rhachodesmidae
    |--Aceratophallus dux Chamberlin 1914K54
    |--Elcarmenia Kraus 1954K54
    |    `--*E. engelhardi Kraus 1954K54
    |--Pammicrophallus Pocock 1909K54
    |    |--P. ornatusK54
    |    `--P. pasaquinae Kraus 1954K54
    |--Neoleptodesmus Carl 1903K54
    |    |--N. aztecusK54
    |    |--N. elevatusK54
    |    |--N. geniculatus Kraus 1954K54
    |    |--N. guatemalensisK54
    |    |--N. orizabaeK54
    |    |--N. salvadorensis Kraus 1954K54
    |    |--N. sinuatus Kraus 1954K54
    |    |--N. sumichrastiK54
    |    |--N. vermiformisK54
    |    |--N. vertebratusK54
    |    `--N. vulcani Kraus 1954K54
    |--HolistophallusLH62
    |--Tancitares Chamberlin 1942H11
    |    `--*T. michoacanus Chamberlin 1942H11
    `--Pararhachistes Pocock 1909H11
         |--P. amblus Chamberlin 1942H11
         |--P. elevatusH11
         |--P. galeanae Chamberlin 1943H11
         |--P. potosinus Chamberlin 1943H11
         `--P. vertebratusH11

*Type species of generic name indicated

References

[H11] Hoffman, R. 2011. The curious genitalic structures of Tancitares michoacanus (Polydesmida, Rhachodesmidae). International Journal of Myriapodology 5: 27–33.

[K54] Kraus, O. 1954. Myriapoden aus El Salvador. Senckenbergiana Biologica 35 (5–6): 293–349.

Loomis, H. F. 1964. The millipeds of Panama (Diplopoda). Fieldiana: Zoology 47 (1): 1–136.

[LH62] Loomis, H. F., & R. L. Hoffman. 1962. A remarkable new family of spined polydesmoid Diplopoda, including a species lacking gonopods in the male sex. Proceedings of the Biological Society of Washington 75: 145–158.

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