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Aquatic Invertebrates

Eastern Dobsonflies Emerging

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Eastern Dobsonfly (Corydalus cornutus) larvae, known as Hellgramites, are the top invertebrate predators in the rocky streams where they occur. In this stage they look like underwater centipedes and consume tadpoles, small fish, and other young aquatic larvae. Adults keep watch over them from a nearby area above the water.

After leaving their stream and pupating on land, the 4”-5 ½”-winged adults, referred to as Eastern Dobsonflies, emerge. Males can easily be distinguished from females by their large, sickled-shaped mandibles which the females lack. (The short, powerful mandibles of the female are capable of giving a painful bite, which the males’ mandibles are not.) Adults are primarily nocturnal and they do not eat. During their short lifespan (about three days for males, eight to ten days for females) they concentrate on reproducing.

The elongated jaws of the male are used both as part of the premating ritual (males place their mandibles on the wings of the females) and as weapons for fighting rival males.   (Thanks to Clyde Jenne for photo op.)

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Broad-shouldered Water Striders Still Active

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If you see what look like miniature water striders skating on the surface of a stream or pond, you may have come upon an aggregation of Broad-shouldered Water Striders, a different family of water striders from the ones we commonly see. They are tiny (2-6 mm) and very fast-moving, zipping here and there with the speed of a bullet, staying on top of the surface film, or surface tension, that is created by the attraction of water molecules. Adaptations to this mode of travel include non-wettable hairs at the ends of their legs that don’t disrupt the surface tension, and claws that are located a short distance up the outermost section of their legs rather than at the end of their legs, so as not to break this film.

Broad-shouldered Water Striders are often found in the more protected areas of a stream, where they tend to congregate in large numbers. Members of a common genus, Rhagovelia, are known as “riffle bugs” and are often found below rocks that are in the current. Broad-shouldered Water Striders locate their prey (water fleas, mosquito eggs and larvae, etc.) by detecting surface waves with vibration sensors in their legs. There can be up to six generations a summer (photo shows that they are still mating at the end of October). Broad-shouldered Water Striders spend the winter hibernating as adults, gathering in debris at the edge of the water or beneath undercut banks.

 

 


Crane Fly Larvae

8-3-17 crane fly larva 049A1624A favorite past-time of mine is peering under logs and rocks to see what might be living there. (The logs and rocks are carefully replaced in the position in which they were found so as not to disturb the inhabitants any more than is necessary.)  Recently I discovered a Crane Fly larva under a rock that was adjacent to a stream – a typical spot in which to find a soon-to-pupate larva.  Roughly an inch long, the most distinguishing features are the ridges along its body, and the star-like appendages at the tip of its abdomen. If you examine the appendages closely, you will find two spiracles, through which the Crane Fly larva breathes, located in a recessed area in the center. Although you can’t see a head, it has one that is tucked into its thorax.

The Crane Fly family is the largest family of true flies, in terms of number of species.  They can be found in aquatic, semi-aquatic and terrestrial habitats. True flies do not have segmented legs that they can use to hold on to the substrate when they catch their prey.  Many Crane Fly larvae feed on decomposing leaves, but those species of Crane Flies which are predators are capable of forming a large knot with the muscles at the end of their abdomen.  When they catch prey with their mouthparts, they enlarge the end of their abdomen and wedge the knot between stones  in order to anchor themselves.