Hunting the Hunter (Part 2)

Hunting the Hunter (Part 1) was written about a year ago – you can read it here. That post was mostly about spider-hunting wasps of the family Pompilidae, and finished with the image below as a bit of a teaser to this second part.

spider fly

The image shows several fly larvae (the arrow-headed segmented hairy things) and lots of eggs (the black ovoid things). They are the eggs and larvae of flies from the family Acroceridae. Adult flies of this family may be known as ‘bladder flies’, ‘small-headed flies’, ‘hump-back flies’, or ‘spider flies’. The image below indicates how a couple of those common names may have come about.

bladder fly

Acrocerid flies are well known for having enlarged compound eyes that meet in the middle of their head, which probably provides an extended field of vision. The adults of some species have elongated proboscises, sometimes longer than their entire body. The individual pictured here is of the genus Ogcodes which has a very short proboscis.

The larval stages of acrocerid flies are endoparasitoids (i.e. internal parasites) of spiders. After a pair of acrocerid flies have mated, the female flies deposit tiny eggs en masse on dead twigs, grass stems, fences, wires, and other structures. The eggs are so small the egg mass looks like soot at first sight (as you can see in the image below). There may be as many as 5000 ‘microtype’ eggs in each egg mass – all of which may hatch into spider-seeking larvae.

Ogcodes eggs

First instar larvae (such as the ones in the image at the top of this post) are free-living planidia which actively seek out suitable spider hosts. The spines that you can see on the larvae must really help when climbing onto a hairy spider.

Once an acrocerid larva finds a spider, it burrows inside, attaches to the spider’s book lungs, and begins feeding on its body fluids. The larva moults three times, growing larger each time, feeding all the while. The final instar (growth stage) larva kills the spider just prior to emerging from it. The acrocerid larva consumes the entire contents of the spider’s body, leaving an empty exoskeleton. The larva then pupates, and an adult acrocerid fly eventually emerges from the pupa. The image below shows a male (left) and a female (right) of the genus Ogcodes.

Acroceridae

Many people think of flies as being either dirty (as in the case of house flies Musca domestica) or annoying (bush flies Musca vetustissima). There is much more to flies than that! There are about 7000 species of flies described in Australia so far, and there may be as many as 30,000 species here. If acrocerid flies are anything to go by, there must be lots of flies with amazing life cycles that we don’t know about yet!

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2 Replies to “Hunting the Hunter (Part 2)”

  1. Were these photos taken in Pomonal? Are there acrocerid flies here helping to control my spider population? And do they favour certain spider species? Huntsmen? Red Backs?

    • The images were taken in the Wartook valley. Not much work has been done on these flies in Australia, so I am not sure of specific hosts here.

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