Slender Robber Fly - Leptogaster sp.

Family Asilidae, Subfamily Leptogasterinae

This page contains pictures and information about Slender Robber Flies in Subfamily Leptogasterinae that we found in the Brisbane area, Queensland, Australia.


Mating pair, body length 15mm, Nov 2008, Yugarapul Park

There is only one Australian genus, Leptogaster, in this subfamily. They are relatively small and very slender.

Subfamily Leptogasterinae is considered as the more primitive group in Robber Flies family. Members in this subfamily are quite different from other Robber Flies. They fly slowly as damselfly. They attack prey that are insects on plants and even spiders on web, but not those flying insects on flight.


Leptogaster sp., body length 15mm

Insects are not always be the prey of spiders. Some insects attack spiders. Robber Fly is one of those insects. On a early summer day afternoon, a medium size Silver Orb Wed spider was building its web. The spider had finished laying the frame threads and radius of the web and about to put on the sticky spiral silk. There was a medium size Robber Fly flying over the spider web. After the fly circled two rounds, it attacked the spider on its abdomen. The fly punch its mouth parts into the spider body. There was only very little struggling, then all become motionless. The Robber Fly start feeding the spider on the unfinished spider web. We believed if the spider web were finished, the Robber Fly may not win so easily.


Leptogaster sp., body length 15mm

Unlike most robber flies in other subfamily, the robber flies in this family do not attack prey during flight. They attack prey that are not on the wings. The pictures show the fly just captured a Katydid nymph.



This fly has the extraordinary long hind legs to grab the prey. Other subfamilies the robber flies have long "beard" of setae around the face. It is believed that those "beard" serve as protection to their face from damage by the prey. Robber flies in this subfamily do not have those long "beard", actually they don't need them. They hold the prey by their extraordinary long hind legs, move the prey to a "safe" position and then punch them with the mouth.

Their tarsus are extraordinary long too. The Slender Robber Flies use their tarsus as our human hand, for holding prey and stems.

Reference:
1. Insects of Australia, CSIRO, Division of Entomology, Melbourne University Press, 2nd Edition 1991, p 758.
2. Insects of Australia and New Zealand - R. J. Tillyard, Angus & Robertson, Ltd, Sydney, 1926, p363.
3. Leptogastrinae page - Rubber Flies.

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