Small-headed Flies (Family: Acroceridae)
Also known as spider or hunchback-flies. Acroceridae is a family of Nemestrinoid flies which boast a cosmopolitan distribution, but interestingly are not abundant anywhere throughout their range. There are ~550 species of acrocerids described which are housed in 3 subfamilies, many but not all of them are bee or wasp mimics. Acrocerids are parasitoids of spiders, their first-instar larvae will seek out and force themselves inside the spider where they usually lodge themselves near the book lungs. They will then complete the rest of their development inside the spider, and when they are ready to hatch they will exit the spider killing it in the process. Adult small-headed flies are more tame compared to their young as they feed on nectar, using a long proboscis which is tucked under their body.
Classification
Animalia-Arthropoda-Insecta-Diptera-Brachycera-Asilomorpha-Nemestrinoidea-Acroceridae
Images: JonRichfield and James Niland