Diptera.info - Articles: Observations on Medetera jacula (Fallén, 1823)

Feeding
It is generally accepted that Dolichopodidae suck out contents of their prey (* see below). To my great astonishment I have found that Medetera never does so, but always swallows its prey (or releases it 'alive and kicking' if swallowing is impossible). At least 90% of Medetera's prey have to be called 'tiny creatures of unknown origin'. The feeding is difficult to observe because it appears only as follows: Medetera's hunting position is often finished by short and fast nod of the fly's head to the bark surface and then one can see during 1-2 sec slight movements of Medetera's horse-like lower part of face. That is all. To detect the origin of such prey one has to look at Medetera at hunting position through powerful macro lens. Figure 2 shows how little time has an observer even in case of Aphanogmus sp. which is not the smallest prey. (To forestall questions I do not know what the matter with fly's thorax is.) In a unique photo with really small prey, the prey is juvenile Collembola, probably Neanuridae - Neanura muscorum (fig. 3).
I have a total of 52 observations with identified prey:

Acarina, Macrochelidae (?)	1
Acarina, Bdellidae	7
Collembola, Neanuridae, Neanura muscorum?	2
Collembola, Entomobryidae, Entomobrya nivalis?	1
Collembola, Isotomidae	8 + 1 = 9
Thysanoptera	1
Homoptera, Aphidoidea	4
Homoptera, Aphidoidea, Callaphididae, Callipterinella tuberculata	1
Hemiptera, nymph Lygaeidae	1
Hymenoptera, Ceraphronidae, Aphanogmus sp.	2
Hymenoptera, Platygastridae (?)	1
Diptera, Ceratopogonidae	1
Diptera, Sciaridae (early August species)	4
Diptera, Sciaridae (late August species)	3
Diptera, Cecidomyiidae	13
Coleoptera, Curculionidae	1

Once again, this list shows nothing except the "visible part of Medetera's menu". I think that Medetera has no group-specialty at all in prey choice; it attacks any sizable moving creature.
Prey marked red were afterwards released as too hard or too large (fig. 4). Bdeliid mites are a suitable size but were released, probably as poisonous, 2-4 sec after capture.

Acarina (Macrochelidae), Collembola (Neanuridae), Hemiptera and Hymenoptera (Ceraphronidae) were swallowed in less than 5 sec. Medetera are very voracious; several times I observed two successful hunts of this kind during one minute.
In case of large prey Medetera usually catches it by the dorsal side of the geometrical center of the prey's body and swallows the prey from hind part to head. The captured prey usually remains alive (moves legs and antennae and hold its body) for not more than 1 minute. Several times Medetera decapitated almost-swallowed Cecidomyiidae (fig. 5), or Sciaridae using its forelegs, and once broke off the furcula of an almost-swallowed Collembola, Isotomidae. Also with the help of its forelegs Medetera releases captured prey which it finds inedible for some reason. I have the impression, that Medetera cannot release prey by simply relaxing its mouthparts but can do it only with the use of its forelegs. Swallowing of aphids took 40-60 sec (fig. 6), but in one case 17 min.
Swallowing of large Collembola took 3-10 min (fig. 7).
Swallowing of Cecidomyiidae usually takes 4-5 min, but may be up to 15 min (fig. 8).
It seems that Medetera's mouthparts work as an external elastic muscular stomach.
I always observed fluid excrements of Medetera as shows fig. 7-5. Thus, it seems that Medetera completely digest prey.

I have never seen Medetera becoming prey for anything else. Only once I met Medetera with parasitic mite on its body (fig. 9).
Medetera's detect its prey from a distance of 0.5 - 2 cm. Frequently I observed that Medetera detects Coccinellidae larvae or Cicadellidae from distance of 5-6 cm, runs to 2-3 cm and from this shorter distance realizes that the prey is too large and stops. So, Medetera has distant vision, but this vision is not keen enough.
The hind quarter of Medetera's field of vision is a "blind sector". If, for example, an ant approaches to Medetera inside this blind sector, Medetera does not jump away, which is what it always does otherwise. This blind sector seems to be important also for courtship.

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#1 | Susan R Walter on 22 January 2007 15:18:06

Really interesting (and amusing) article Nikita. The feeding strategy details were particularly fascinating.

#2 | jorgemotalmeida on 24 January 2007 21:48:36

NIkita, mystery solved! smiley your avatar is a doli fly.
FANTASTIC article. I wish to see more articles like this one.

one day I hope to contribute with 1 article!

OR more.

#3 | diphascon on 24 January 2007 23:19:22

Very, very well done!

#4 | crex on 25 January 2007 17:27:16

Interesting. Well done, although it should have been made as a PDF

#5 | Stephen on 07 February 2007 23:10:19

Fantastic! Observations worth of Fabre.

#6 | Cheryl Moorehead on 14 February 2007 08:46:51

I really liked this article. I've only been aware of these flies since I started macro work. They hunt barklice on my Hemlock.

#7 | Toby on 31 March 2007 23:52:16

fascinating, I had no idea flies eat springtails.

#8 | LordV on 01 April 2007 08:24:11

Wonderful article
Brian V.

#9 | conopid on 02 April 2007 18:38:09

Nikita,
I have only just read this. It is a very impressive example of observation. If only more of us would do this type of thing, we could learn so much more. Well done.

#10 | Juergen Peters on 04 October 2007 16:35:24

Very interesting! I have seen Medetera (most likely (near) this species according to I. Grichanov) here also numerously on tree trunks in the last days, even at night in garden. One behaviour I found remarkeable: when sitting head up at the tree, they tend to lean from left to right and back when disturbed slightly. Looks like they were dancing...

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