Scarabaeus (Scarabaeus) cancer ( Arrow, 1919 )

Harrison, J. Du G., Scholtz, C. H. & Chown, S. L., 2003, A revision of the endemic south-western African dung beetle subgenus Scarabaeus (Pachysoma) MacLeay, including notes on other flightless Scarabaeini (Scarabaeidae: Scarabaeinae), Journal of Natural History 37 (3), pp. 305-355 : 349-350

Scarabaeus (Scarabaeus) cancer ( Arrow, 1919)

( figures 1, 2, 35 , 53, 74)

Mnematium cancer Arrow, 1919: 433 ; Boucomont, 1925: 116; Ferreira, 1961: 26; Balthasar, 1963: 139; Ferreira, 1969: 28.

Pachysoma cancer (Arrow) : Holm and Scholtz, 1979: 227.

Scarabaeus cancer (Arrow) : Mostert and Holm, 1982: 278.

Diagnosis. Clypeus quadridentate, clypeal teeth pointed (both specimens examined are worn, so shape of apex of clypeal teeth cannot be determined); genae extended laterally, anterior edge produced into a point (also worn), posterior edge coarsely serrate, dorsal surface with scattered setiferous punctures ( figure 35 ); protibia quadridentate (no sign of serrations between worn protibial teeth), edge irregular below protibial teeth ( figure 53); profemur enlarged, spined, procoxae with large spines on anterior edge; spurs simple in both sexes; very faint sub-elytral ridge, elytral rim larger, humeral calli absent, anterior edge of elytra rounded, striae faint; metatarsal claws were absent in both specimens, but Arrow (1919) describes tarsi and claws as ‘both pairs of tarsi are rather broad and the claws are minute, short, straight, and not divergent’; mesocoxae contiguous, deep depression separating them; mesosternal ridge absent; male genitalia illustrated in figure 74a, b.

Distribution and habitat. Only recorded from the Kwatiri and Longa rivers in Angola ( Boucomont, 1925) ( figures 1, 2). Based on the distribution of the other flightless Scarabaeini ( figures 1, 2), one would expect this species to occur in the coastal dunes north of the Kunene River, i.e. the Mocamedes Desert. The known distribution of S. (Scarabaeus) cancer suggests rather an association with inland rivers. Sand bodies near these rivers would be a likely place to look for S. (Scarabaeus) cancer .

Comments on locality data. When Arrow (1919) described Mnematium cancer he remarked, ‘A single specimen of the extraordinary insect here represented, bearing no label to record its origin or habitat, was in the collection of the late BG. Nevinson, recently presented to the British Museum by his son. Probably, like its nearest allies, it inhabits the western part of southern Africa, possibly the Bihé district of Angola, from where Mr. Nevinson received other interesting beetles’. As the BMNH collection currently has two specimens, S. (Scarabaeus) cancer was obviously collected again as the female does not have a BMNH locality label. Boucomont (1925) records two localities in the Kubango District of Angola, where S. (Scarabaeus) cancer was collected. The female in the BMNH possibly comes from this expedition.

Morphology. S. (Scarabaeus) cancer is hard to place within the Tribe Scarabaeini . The profemora and procoxae suggest it evolved from a Pachylomerus -like ancestor. The asymmetrical aedeagus and two tarsal claws place it closer to Scarabaeus . While the shape of the head is very similar to S. (Scarabaeus) ritchiei , S. (Scarabaeolus) silenus and the flying S. (Scarabaeus) multidentatus , placing it within the Mnematium / Mnematidium species group. The degree of morphological change that has occurred since wing loss is similar to Pachysoma , i.e. contiguous mesocoxae, mesosternal ridge absent, fused elytra, rounded anterior elytral edge.

Biology. The biology of this species is unknown. However, based on the biology of other Scarabaeini one could expect S. (Scarabaeus) cancer to behave and forage like Pachylomerus femoralis (see Tribe, 1976) or Scarabaeus catenatus (see Sato, 1997, 1998). Both these flying species have very short mesocoxal distances, which is probably an adaptation or pre-adaptation to a multi-foraging strategy, and enlarged profemora. The mouthpart morphology of S. (Scarabaeus) cancer is closest to S. ( Pachysoma ) species, which suggests dry dung feeding or at least both dry and wet feeding. Collected during April and May 1913.

Comments. Arrow (1919) refrained from placing S. (Scarabaeus) cancer in its own genus, because of the unsatisfactory character of several genera within the Scarabaeini at the time. Holm and Scholtz (1979) suggested that if any species warranted a separate genus S. (Scarabaeus) cancer would. Mostert and Holm (1982) discussed S. (Scarabaeus) cancer as incertae sedis but placed it within Scarabaeus sensu lato. The temptation to create a new genus is great, probably because of the large body size and spectacular profemora of S. (Scarabaeus) cancer . However, S. (Scarabaeus) cancer warrants a separate genus no more than S. (Scarabaeus) sevoistra or S. (Scarabaeolus) scholtzi (which are just as unique, but much smaller). While flying species seemingly warranting their own genera include S. galenus , S. multidentatus and S. proboscideus .

Eighty years after the description of S. (Scarabaeus) cancer , a well-supported decision on the generic placement of this species can still not be made. Progress has been compounded by the lack of specimens (only two known) and biological information for S. (Scarabaeus) cancer . With the advent of molecular biology, molecular data (if ever S. (Scarabaeus) cancer is recollected) should be added to the morphological before a decision is made regarding the origin or generic placement of S. (Scarabaeus) cancer . As the last known collection of this species was 86 years ago, and the area where they occur is probably mined, this might never be possible.

Types. Mnematium cancer was described from the single male holotype, housed in the BMNH collection ( Arrow, 1919) .

Type material examined (1 3 diss.). HOLOTYPE 3, ANGOLA: (no type locality, but Bihé (Kuito) District at 12 ° 23 ∞ S, 16 ° 56 ∞ E suspected) / Holotype (white paper disk with a red circle, ‘Holotype’ typed in black) / Nevinson Coll. 1918-14 / Mnematium cancer type Arrow / Aedeagus point mounted / Mnematium cancer 3 Arrow, M.E. Bacchus, det. 1975, Holotype, (1 3 BMNH).

Additional material examined from Angola (1 ♀ diss.) .

Figures

FIGS 18–35. (18–30) Head of Scarabaeus (Pachysoma) species in dorsal view: (18) S. (P.) aesculapius. (19) S. (P.) hippocrates. (20) S. (P.) glentoni. (21) S. (P.) endroedyi. (22) S. (P.) striatus. (23) S. (P.) gapiepinus. (24) S. (P.) bennigseni. (25) S. (P.) schinzi: (a) female, (b) male. (26) S. (P.) valeflorae: (a) female, (b) male. (27) S. (P.) fitzsimonsi. (28) S. (P.) rotundigenus. (29) S. (P.) rodriguesi. (30) S. (P.) denticollis. (31, 32) Head of flightless Scarabaeus (Scarabaeolus) species in dorsal view. (31) S. (Scarabaeolus) scholtzi. (32) S. (Scarabaeolus) silenus. (33–35) Head of flightless Scarabaeus (Scarabaeus) species in dorsal view. (33) S. (Scarabaeus) ritchiei. (34) S. (Scarabaeus) sevoistra. (35) S. (Scarabaeus) cancer.