One of my favorite insects is quite uncommon (described as rare), but can be found if you know when and where to look for them! Jewelled Spider Flies currently can be seen along Mary Bowerman Fire Interpretive Trail (western and northern side) and at some other locations. They are amazingly-colored, with metallic blue, purple or green colors (hence they are termed jeweled), and have some of the most incredible mouthparts in the insect world! The flies we see in Mount Diablo State Park, in genus Eulonchus, have a super-long proboscis, and there are at least 2 species present.  The proboscis is typically longer than the bodies of the flies, and they fold it underneath their body as they fly!  These are beautifully metallic flies with a truly fascinating life history. Their antennae are inserted near the middle of the head and their eyes cover much of the head.

Flies in the genus Eulonchus attack spiders in the families Euctenizidae (Wafer-lid trapdoor spiders) and Antrodiaetidae(folding trapdoor spiders, a small spider family related to atypical tarantulas). Their larvae are specialized endoparasitoids of spiders (endoparasitoids develop within the body of another animal). The adult female lays eggs, sometimes thousands of them, which then hatch into larvae. First stage Acrocerid larvae are free-living planidia that move similarly to an inchworm. Most fail miserably in finding a host. The lucky ones find one of these relatively rare spiders either by wandering and actively seeking out a spider host or by sitting and waiting for the host to pass by. The larva will attach to a spider, enter into an opening, and attach at or near the book lung of the spider. Over a long period of time, it will feed on the spider from the inside, eventually killing it.

Look for these flies now, and in the coming few weeks, flying around Red Ribbons flowers (Clarkia concinna) and Bunchleaf Penstemon flowers (Penstemon heterophyllus). I also have seen them on Ithuriel’s Spear flowers (Triteleia laxa), but in fewer numbers.