Some vertebrate parasites have complex life cycles that require an invertebrate host. For the rat tapeworm, Hymenolepis diminuta, the intermediate hosts are insects. One host of the rat tapeworm is the broad-horned flour beetle, Gnatocerus cornutus.
The rat tapeworm does not kill broad-horned flour beetles but it affects their nutrition and size. Jeffery Demuth, Amrita Naidu, Laura D. Mydlarz writing in the Journal, PLoS One, describe the effects of rat tapeworm infection on growth and reproduction of male broad-horned flour beetles. Male larvae infected with the tapeworm parasite have smaller mandibles when they emerge as adults.
In these beetles, the males have enlarged mandibles that are used in mate competition with other males. The male beetles will face off, lock mandibles and push the other males like Sumo wrestlers. The enlarged mandibles (horns) can be used by one beetle to lift the other into the air. The mandibles can also be used to bite. After the contest of strength, the winning male will mate with any females within his territory. The loser will become more sedentary and will not attempt to contact or interact with females.
Many species are characterized by a range in sizes of morphological features such as mandibles. When taxonomists study a species, they often note quantitative differences in traits, but without further study, it is difficult to determine whether the trait differences are due to genetic variation or to environmental factors. Tapeworms and other parasites are important environmental factors that affect insect traits and appearance.