Cryptoxyleborus Schedl, 1937

Beaver, Roger A. & Hulcr, Jiri, 2008, A Review of the Ambrosia Beetle Genus Cryptoxyleborus Schedl (Coleoptera: Curculionidae: Scolytinae), The Coleopterists Bulletin 62 (1), pp. 133-153 : 133-153

Cryptoxyleborus Schedl

Cryptoxyleborus Schedl, 1937:550 .

Type species: C. naevus Schedl , designated by Schedl, 1963:103.

Diagnosis. Body length from 1.5–4.8 mm, elongate, length to width ratio 2.7– 3.4. Antennal funicle 5–segmented; antennal club usually of type 3–4, but of type 1 in one species–group ( Fig. 4 ). Eyes often very large, coarsely facetted, extending onto frons, which is accordingly narrowed. Pronotum longer than wide (length to width ratio 1.1–1.3), usually of type 8 in dorsal view ( Fig. 5 ), but types 6, 7 and 9 occur; in lateral view type 7–8; anterior margin either unarmed or with 2–8 small asperities; anterior slope convex, armed by many small, transverse asperities; summit anterior to middle; disc usually shining and finely punctured, without a mycangial hair tuft; lateral margin either rounded or bluntly costate. Scutellum hidden by elytra, small, laterally compressed, pointed. Elytra elongate, 1.4–2.0 times as long as pronotum, strongly tapering posteriorly and often prolonged into a point; elytral mycangia present, opening on anterior slope of elytral base, or on dorsal elytral surface close to base; elytral bases variable depending on position of mycangia, usually straight, but curved and carinate when mycangia open dorsally; elytral disc usually shining, either seriate–punctate with one or two rows of interstrial punctures, or punctures confused, especially near elytral base and apex; elytral declivity without a lateral costa or carina; elytral sculpture without strong tubercles, but sometimes with hooked teeth, often granulate toward elytral apex; elytral vestiture variable from almost absent except close to apex, to moderately dense and long, extending to base. Legs with procoxae contiguous, with occasional exceptions, protibia with posterior surface more or less inflated, and usually granulate, almost always with 5 or 6 socketed teeth on lateral margin; meso– and metatibiae compressed. Abdomen strongly tapering posteriorly, ventrites 1 + 2 usually as long as or longer than ventrites 3–5 together.

Cryptoxyleborus can be distinguished from other xyleborine genera by the combination of elongate pronotum, concealed scutellum, elytral mycangia and strongly tapering elytra. Some species of Xyleborus have similarly tapering elytra, but have a differently shaped pronotum, a normal scutellum, and lack elytral mycangia. Certain species of Microperus , such as M.nanus , with type 1 antennae, a concealed scutellum and elytral mycangia, are very similar to some small species of Cryptoxyleborus , but are distinguishable by the different shape of the pronotum, the less strongly tapering elytra with a more rounded apex, and the flattened protibia.

Sexual Dimorphism. Like all other xyleborine genera, Cryptoxyleborus is sexually dimorphic, the females diploid, the males haploid (Jordal 2002). The males of only five species have been described: C.confusus Browne ( Browne 1962) , C. eggersi Schedl ( Schedl 1942, as C. dryobalanopsis Schedl ), C.naevus ( Browne 1972) , C.subnaevus ( Schedl 1966) , and C. turbineus (Maiti & Saha 2004) . Unlike the males of some xyleborine genera, but as in Cyclorhipidion Hagedorn , they are mostly well–built and strongly sclerotised. However, they have smaller eyes, lack mycangia, and the hind wings are greatly reduced and functionless. The sculpture of the elytra is reduced. The elytral apex and the fifth abdominal ventrite are much more broadly rounded in the male than the female. The relative proportions of the pronotum and elytra differ between male and female. The males of C. confusus , C. naevus and C. subnaevus are as large or almost as large as the females, but in C. eggersi and C. turbineus , the male is only 0.75–0.8 times the length of the female. The overall ratio of length to width of the body is similar in both sexes, but the male pronotum is usually longer relative to its width than in the female, and the elytra are shorter relative to the length of the pronotum. The ratio of elytral to pronotal length is 1.1–1.3 in the male, but 1.5–1.7 in the female. The males tend to be more variable in size and proportions than the females, perhaps as a result of their haploidy.

Distribution. The genus is widely distributed from India to New Guinea. Most of the species are found in the rain forests of southeast Asia and Indonesia, with only two species occurring in northeastern India, and only two species east of Wallace’s line, in Sulawesi and New Guinea. This distribution coincides with the tropical Asian and Papuan range of the host trees of the family Dipterocarpaceae . The highest number of species (12) is found on the island of Borneo (including the political units Brunei Darussalam, East Malaysia and Indonesia (Kalimantan )), with 8 species recorded from West Malaysia. It is curious that no species has been recorded from Java, despite extensive collecting there by forest entomologists, particularly L. G. E. Kalshoven. A single record of Cryptoxyleborus subnaevus from Australia probably represents an introduced species.

Biology. All species of Cryptoxyleborus are ambrosia beetles breeding in the wood of dead or dying trees of the family Dipterocarpaceae . The gallery system ( Browne 1961) usually consists of an unbranched entrance tunnel leading to a single terminal brood chamber, enlarged in the longitudinal plane. Browne (1961) notes a variant on this pattern in C. confusus Browne , in which the brood chamber occurs between bark and wood, but the gallery system also penetrates the wood, and may branch there. Whether the flat brood chamber is constructed by the female or by the progeny has not been directly observed, but it is likely analogous to the situation in other representatives of related genera such as Xyleborinus , where the flat chamber is a result of the larval communal feeding on a mixture of xylem and fungus tissues (xylomycetophagy sensu Roeper 1995). The eggs are laid and larval and pupal development occurs in the brood chamber. The fungus involved has not been studied. As in other xyleborine genera, the sex ratio is strongly skewed towards the female sex, apparently with only a single male developing to maturity in each gallery. The male mates with his sisters in the brood chamber, the newly mated young females emerge through the original entrance hole and disperse to find new breeding sites. There is normally only a 136 single generation in a single log. By the time that a new generation has developed, the log is no longer suitable as breeding material. Brood sizes seem to be quite large. Browne (1961) records a brood of C. simplex including 50 individuals (together with a predatory clerid larva that may already have reduced brood size), and broods of C. confusus ranging from 18–39 individuals. The species of Cryptoxyleborus are normally found together with other species of scolytine and platypodine ambrosia beetles attacking the same logs. None of the Cryptoxyleborus species is common in collections, and some are known only from one or a few individuals. Most ambrosia beetles are not host–specific (Beaver 1979; Hulcr et al. 2007 b). They tend to attack any hosts in which the symbiotic ambrosia fungus can grow and provide food for the larvae. However, species of Cryptoxyleborus have been only recorded from trees in the family Dipterocarpaceae . Such host specialization is most unusual in ambrosia beetles, but also occurs in the platypodine ambrosia beetle genera Genyocerus Motschulsky , and Pereioplatypus Beaver ( Browne 1961; Beaver 2007; Beaver & Liu 2007), and in the scolytine genus Webbia Hopkins ( Browne 1961) , in all cases on Dipterocarpaceae , and in a few species in other genera of both scolytines and platypodines. In the very diverse tropical rain forest, it is easier for specialization of ambrosia beetles to evolve on trees which are either abundant and/or produce resin or latex with volatile chemicals which are attractive to the beetles (Beaver 1979). When it is necessary to find a new breeding site in each generation, and suitable breeding sites are likely to be scattered in both space and time, these qualities make host–finding easier for the beetles. Because the species are uncommon, and their gallery systems do not penetrate deeply into the wood, none are known to be of any economic importance.

Figures

Fig. 4. Formalized types of antennal club found in Xyleborina. After Hulcr et al. (2007a). Upper row - 5 basic types; second row - variations of the types 1–3; third row - posterior face of the club of types 1 and 2; fourth row - lateral view of types 1–3.

Fig. 5. Formalized types of pronotum shapes found in Cryptoxyleborus, dorsal and lateral aspect. For the complete set of shape types in Xyleborina see Hulcr et al. (2007a). The dorsal types are characterized by length/width ratio and by the lateral expansion of the anterior part of the pronotum; the lateral types differ in length/height ratio.