Different Jaws for Different Jobs

Arrhenodes minutus (oak timberworm) | Wayne Co., Missouri

If you’re interested in wood boring beetles and live in the eastern  U.S. like I do, you’re sure to encounter sooner or later the region’s sole¹ “primitive weevil” (family Brentidae), the oak timberworm (Arrhenodes minutus).  This beetle develops as a larva in the wood of living trees exposed by wounding, creating numerous small “worm holes” that can occasionally degrade the value of wood grown for timber.  Females are presumably attracted to volatiles given off by wounded wood for oviposition, thus they are also commonly attracted to the trunks and stumps of trees harvested for lumber or cut for some other reason.  Cut trees are also highly attractive to wood boring beetles in the families Buprestidae and Cerambycidae—my primary taxa of interest, so I’ve seen more than a few oak timberworms over the years, including this male and female that I found on the cut stump of a large black oak (Quercus velutina) in Sam A. Baker State Park, Wayne Co., Missouri.

¹ Actually, there are three other species in eastern North America as well, but all are Neotropical species that occur no further north than the southern tip of Florida (Thomas 1996).

Female - beak thin and elongate

Male - beak short w/ robust mandibles

An interesting feature of oak timberworms and related species of primitive weevils is the rather extreme sexual dimorphism exhibited in the shape and function of the mandibles.  Mandibular sexual dimorphism is actually quite common across many groups of beetles, but in most cases the males simply have proportionately larger mandibles than females due to their use in sexual combat (think stag beetles, for example).  Oak timberworm males also have enlarged mandibles for combat with other males (males are territorial and guard females during oviposition).  The females, however, rather than simply having smaller yet similarly shaped versions of the male mandibles, instead have tiny little mandibles at the end of a greatly narrowed and elongated rostrum (beak).  This is because, unlike most other beetles in which the female mandibles lack a specific purpose, female oak timberworms use their mandibles to “drill” holes into the wood in which they will insert their eggs.  Different forms for different functions!

Mate-guarding behavior

I have read reports of males assisting females in removing her beak if stuck in the wood while drilling an egg hole by “stationing himself at a right angle with her body and pressing his heavy prosternum against the tip of her abdomen, her stout fore legs thus serving as a fulcrum and her long body as a lever” (Riley 1874, as quoted in Thomas 1996), making this a rare instance of tool use by insects.  I have not observed this behavior myself, but it is common to find the males in various mate guarding positions over the female as pictured above.


Riley, C. V. 1874. The northern brenthian—Eupsalis minutus (Drury). (Ord. Coleoptera; Fam. Brenthidae). Sixth Annual Report on the Noxious, Beneficial, and Other Insects, of the State of Missouri. Began and Carter, Jefferson City, Missouri, 169 pp.

Thomas, M. C. 1996. The primitive weevils of Florida (Coleoptera: Brentidae: Brentinae). Florida Department of Agriculture & Consumer Services, Division of Plant Industry, Entomology Circular No. 375, 3 pp.

Copyright © Ted C. MacRae 2011

19 thoughts on “Different Jaws for Different Jobs

  1. Strikingly handsome beetles, but your male and female are similar in body size. So the length ranges for the oak timberworm in Thomas (1996), 13-35 mm, do not refer to sexual size dimorphism? That must be an incredible range in body mass for individuals of one species. Applying Hutichinson’s coefficient, the largest beetles would be about 4 times the body mass of the smallest. Two of the three south Florida species have similar ranges (I’d love to see a 2 inch Brentus anchorago). Is such a range in adult size common in wood borers?

    Also interesting that you use ‘combat’ and Thomas uses ‘battle’ as in ‘battles between males sometimes last 10 minutes or more’. Must be impressive if two males contesting a female conger up metaphors of human warfare.

    • Variability in body length is common among wood boring beetles, as differences in host quality/quantity greatly affect larval growth. It seems to impact certain species/groups more than others – probably a reflection of greater variability in the particular host sources they utilize.

      I could have used the term ‘agonistic interaction’, but I fear I’d start losing readers! 🙂

  2. aw, that’s so sweet! chivalry.

    I also came across something akin to male chivalry, tho’ clearly their DNA was talking, in marine isopods. Well, the terrestrial ones which live at the upper edge of the intertidal zone. if they are, hm, shall we say, “on a date” and a wave is approaching, the male will pick up the female, and scurry to safer grounds to avoid getting battered by the wave.


  3. Pingback: Sexual dimorphism in Brentidae weevils.

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