Look carefully. Don’t let the flat body and lack of wings fool you – it’s not a spider. It’s a bat fly.
Bat flies don’t look much like horse flies or house flies, but they’re part of the insect order Diptera, just like their more familiar (if distant) cousins. But instead of scavenging garbage, these flies eat just one thing: bat blood. And they’ve evolved a specialized body shape that helps them cling to the fur or wing membranes of their hosts.
But living on a host that eats insects is dangerous when you're a fly: bats aren't at all squeamish about munching the bat flies they find when they're grooming their fur. Adult bat flies can scuttle quickly out of the way of an approaching tongue, but baby flies (ie: maggots) aren't exactly known for being nimble. Enter one of the strangest things about these flies: they're viviparous. A female bat fly has only one maggot at a time, and it grows inside her body until it's ready to pupate. At that point, the female deposits the larvae onto a wall near the bats' roosting area. The larvae immediately pupates, and stays stuck to the wall while the fly metamorphoses. After the adult fly emerges it crawls off to find another bat, and a lifetime of meals.
References: Dick, C.W. & B.D. Patterson. 2006. Bat flies - obligate ectoparasites of bats. Pp. 179-194 In Micromammals and macroparasites: from evolutionary ecology to management (S. Morand, B. Krasnov, and R. Poulin, eds.). Springer-Verlag, Tokyo
Photo by Carl W. Dick.