While looking for longhorned beetles in the genus Crossidius on flowers of yellow rabbitbrush (Chrysothamnus viscidiflorus) in southern Utah, I encountered one particular plant with numerous blister beetles (family Meloidae) on its blossoms. The orange color, two black pronotal spots, and distinctive black and white longitudinal elytral stripes leave no doubt as to its identity—Pyrota bilineata, but for good measure I sent a photo to my field mate for the trip, Jeff Huether, who confirmed its identity. I had seen singletons of this species at a few previous localities during the trip, so I was intrigued by the large numbers of individuals congregated on this single plant. As I looked at them, I saw one individual that appeared to have something stuck to the tip of its abdomen. I peered closer to get a better look and, to my surprise, discovered that it was actually a male in the act of mating. The male was tiny, only one-third the size of the female, representing about as extreme a size difference in mating insects as I’ve ever seen.
Many species of blister beetles exhibit tremendous size variability, and a unique aspect of some species’ mating behavior is the cantharidin-packed spermatophore produced by males and transferred to females during mating. (Cantharidin is a toxic defensive compound that serves as a very effective deterrent to predation.) The spermatophores are energetically “expensive” to produce and are transferred to females during relatively short-lived mating aggregations. Mating in some species may take up to 24–48 hours, thus reducing the opportunities for multiple matings, and as a result males of long-mated species end up investing rather heavily in a limited number of females compared to males that mate more often. These features lead to size assortative mating (Alcock & Hanley 1987), with males showing a preference for larger females (that are presumably more fecund) and females preferring larger males to maximize the amount of cantharidin that they receive or to ensure receipt of a spermatophore large enough to fertilize their full complement of eggs. Medium-sized individuals, likewise, would choose the largest of the remaining individuals, leaving the smallest individuals to mate among themselves. Alcock & Hanley (1987) also note, however, that not all species of blister beetles exhibit size assortative mating, even though they form large mating aggregations and individuals vary greatly in size. I have not seen any reference to size assortative mating in Pyrota bilineata; however, this example seems to suggest that the behavior is not practiced by this species. This could be due to shorter mating times (leading to more opportunities for mating) or a range of variation in body size that is not sufficient to consistently favor the behavior.
Alcock, J. & N. F. Hadley. 1987. Assortative Mating by Size: A Comparison of Two Meloid Beetles (Coleoptera: Meloidae). Journal of the Kansas Entomological Society 60(1):41–50 [preview].
Copyright © Ted C. MacRae 2013