Friday, August 8, 2014

Mini-mantises: Ochthera Shore Flies

Mantids (aka "praying mantises") are widely acclaimed for having the strongest grip among predatory insects, but many other insects have similar modifications to their front legs that afford them a vise-like purchase on struggling prey. Among the more unusual of these are shore flies in the genus Ochthera, found over most of North America.

I was delighted to discover some of these flies recently here in Colorado Springs, where they favor very shallow, trickling streams with algal mats and other debris they can run around on in search of other small insects.

Ochthera are small, only 4-5 millimeters in body length. They are squat and compact, with triangular faces and somewhat protuberant eyes. Their most obvious feature is the pair of front legs, with an enlarged coxa (segment connecting the leg to the thorax), greatly enlarged and muscular femur ("upper arm" if you will), and curved, blade-like tibia ("forearm" comparison to a person). The tarsi ("feet") are of normal appearance. The forelegs are thus raptorial, meaning heavily modified to sieze prey. Spines and tiny teeth on the inside of the femur help anchor the victim between femur and tibia.

The flies easily overpower other small insects such as midges, mosquitoes, and leafhoppers that alight on the shore or the surface of the water. They can also unearth midge larvae from the muck along the shore.

Watch one of these flies and you will see it periodically "stretch," reaching out, flexing, and waving those front legs. This may be a threat display directed at other members of its own species, or a means of recognizing each other and avoiding cannibalism (Simpson, 1975). Females may literally lash out at males attempting to mate with them. Males attempt to mount females from the rear, jumping on top of them and gripping the female's "shoulders" if they are not rebuffed. He sets the mood by rapidly tapping the sides of her abdomen with his hind feet. Actual mating can last at least five minutes, at least in one species (Deonier, 1972).

Mated females lay eggs singly, usually on dead, water-logged, or partly submerged grass stems at the shore or in the shallows. Larvae of Ochthera are aquatic or semi-aquatic, and likewise predatory; they feed mostly on midge and mosquito larvae that they coil around like a snake constricting a rat. They have mouthparts that penetrate the exoskeleton of their prey and feed on soft internal tissues. Larvae progress through three instars (an instar is the interval between molts), before pupating. The pupal stage is also aquatic, equipped with breathing tubes. The larval stages last an average of 7-11 days, the pupal stage about 7-10 days. Egg to adult thus takes about 16-21 days, at least under laboratory rearing conditions.

Look for Ochthera adults from the end of March through the end of October in appropriate habitats. There are thirteen species known for North America, collectively distributed over most of the continent.

Sources: Deonier, D.L. 1972. "Observations on Mating, Oviposition, and Food Habits of Certain Shore Flies (Diptera: Ephydridae)," Ohio J. of Sci. 72(1): 22-29.
Simpson, Karl W. 1975. "Biology and Immature Stages of Three Species of Nearctic Ochthera (Diptera: Ephydridae)," Proc. Entomol. Soc. Wash. 77(1): 129-155. This is an excellent, well-illustrated paper.
Winkler, Isaac. 2011. "Insect of the Week - number 57," Insect Museum, North Carolina State University.

2 comments:

  1. What unexpected burliness on a fly--interesting stuff. I suppose that mean the ambush bug I've been seeing is similarly strong in the forearms; they are seriously bulked up!

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    1. Indeed, ambush bugs can snap their leg segments together with enough force as to be audible.

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