Literature Review: Invertebrate Biology

by Carl Strang

This week’s literature focus is on insects.

Dragonflies today are tiny compared to the giants of the Paleozoic. The following study looked at why they were so big then.

Dragonflies today are tiny compared to the giants of the Paleozoic. The following study looked at why they were so big then.

Matthew E. Clapham and Jered A. Karr. Environmental and biotic controls on the evolutionary history of insect body size. Proceedings of the National Academy of Sciences, June 4, 2012 DOI: 10.1073/pnas.1204026109

They reviewed maximal insect fossil wing lengths with respect to atmospheric oxygen, and found a positive correlation until the early Jurassic, when birds or flying dinosaurs and perhaps pterosaurs may have forced the increased maneuverability allowed by smaller body size. This came out of the observation that the enormous dragonflies and other invertebrates of the Paleozoic Era existed at a time when atmospheric oxygen was much higher than it is now. They were testing this hypothesis by correlating insect size with atmospheric oxygen. This study points out that freedom from aerial predators was another factor.

Blue-fronted dancer, brown female. In many damselflies, some females are colored differently from males, like this one, while other individuals have male-like colors. The following study looked at why.

Blue-fronted dancer, brown female. In many damselflies, some females are colored differently from males, like this one, while other individuals have male-like colors. The following study looked at why.

Iserbyt A, Van Gossum H, Stoks R (2012) Biogeographical Survey Identifies Consistent Alternative Physiological Optima and a Minor Role for Environmental Drivers in Maintaining a Polymorphism. PLoS ONE 7(2): e32648. doi:10.1371/journal.pone.0032648

They studied female color polymorphs in Canadian sedge sprites (in northeast Illinois found mainly in McHenry County). Their background review indicates that there has been considerable interest in damselfly female color variants. There is evidence in various studies for influences of environmental conditions such as temperature, but also avoidance of male harassment (accomplished by mimicking male coloration). Their results point to a tradeoff between investment in flight muscles vs. immune system, and suggest that both variants deal with male harassment in different ways. Those with an immune system emphasis are more male-like in coloration (presumably they have less problem with diseases and parasites, but then need to minimize male harassment by looking more “butch”), while those with flight muscle emphasis have colors more distinct from male colors (stronger fliers are better able to escape persistent males).

Lang, Michael, et al. 2012. Mutations in the neverland gene turned Drosophila pachea into an obligate specialist species. Science 337:1658-1661.

They found a few base pair substitutions that distinguished this fruit fly species from its near relatives. These changes render the flies unable to take cholesterol and transform it into essential hormones, but they can get replacement building blocks from their cactus host plant. This is an example of one way in which an insect species can become a specialist dependent upon its host.

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