Observations on Medetera jacula (Fallén, 1823)
Posted by Paul Beuk on 22 January 2007 08:47:25
Nikita Vikhrev

M. jacula, a painfully common late summer-autumn species?

Jere Kahanpaa (Diptera.info)
Being very common, large and comparatively easy to identify Medetera jacula presents a good opportunity to observe Medetera behaviour.
During the period 2 August -15 September, 2006 I observed the life of Medetera jacula (= Medetera in text) on an old Salix alba growing on pasture near a small pond, Moscow region, near Dmitrov town, for a total of 30 hours.
Medetera was numerous on chosen trees during all periods of observation; often I could see 3-8 flies at once. In mid September Medetera suddenly disappeared; on 9 Sept they were still common; on 14 Sept I found only 3 specimens; on 15 Sept not one.
On the trunk they concentrate mostly between 0 and 1 m high, but some flies may be seen up to a height of 3 m. It seems that for Medetera it does not matter whether it is a sunny or shadowy part of trunk, but when the weather is rainy, they prefer (not exclusively) sites of dry bark. During rain I found more flies at a height of 1-2 m. Cold and rainy weather does not disturb Medetera activity. Also Medetera could be found on the grass around the tree. I have seen both sexes in approximately equal proportion. Medetera are active till deep twilight; once I even observed a hunting fly 15 minutes after astronomical sunset in cloudy evening.

Most of the time Medetera sits motionless on the trunk, looking towards sky with abdomen down and head up. This position seems to be on one hand most ergonomic, on the other hand it gives highest possible field of view. Let it be called 'waiting' position.
When detecting the prey Medetera changes position into 'hunting' one, with the body parallel to the trunk. Thus Medetera is ready to attack or run after the prey along the trunk. If the prey stops moving, Medetera freezes in hunting position too and leaves freezing for 5-20 sec.
When eating the prey, the fly again uses waiting position. I very often observed the cleaning procedure immediately after feeding. Females in most cases regard head cleaning to be enough (fig. 1), males on the other hand seem to be more tidy, but they do not regard head cleaning of such importance, being much more concerned of hygiene of their genitalia.
For overnight they seem to use a special position with the body parallel and pressed to the bark.

< - - - Please note: There are another three pages in this article! - - - >
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, Bdellidae7
Collembola, Neanuridae, Neanura muscorum?2
Collembola, Entomobryidae, Entomobrya nivalis?1
Collembola, Isotomidae8 + 1 = 9
Homoptera, Aphidoidea4
Homoptera, Aphidoidea, Callaphididae, Callipterinella tuberculata1
Hemiptera, nymph Lygaeidae1
Hymenoptera, Ceraphronidae, Aphanogmus sp.2
Hymenoptera, Platygastridae (?)1
Diptera, Ceratopogonidae1
Diptera, Sciaridae (early August species)4
Diptera, Sciaridae (late August species)3
Diptera, Cecidomyiidae13
Coleoptera, Curculionidae1
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.
The first step of courtship is the male's "wing helicoptering". The wing helicoptering point is to the left or right side (+/- 45 degree) of the object of courtship, at a distance of 1-3 cm. Wing helicoptering is not a nessesary step; it was rare in early August and much more frequent in early September. It takes 2-6 sec.

Then Medetera jumps into blind sector of another Medetera. It is the position for the second step of Medetera's courting. The second step consists of tender stroking of scutellum and wings of the upper fly by one of the forelegs of the lower fly (fig. 11), then less tender stroking done by both forelegs (fig. 12), then mating (or mating attempt) (figs. 13-1, 13-2).

Copulation is short and takes 10-25 sec. Being aware of mating habits of related Empididae family, I was not very surprised to discover that neither female feeding disturbs male mating, nor male mating disturbs female feeding (fig. 14).
As you, of course, noticed there is one problem: the lower Medetera is always a male, but the upper one is in most cases male too!
I imagine, that for male far vision, the tree-trunk landscape with the vague silhouette of another Medetera looks like that shown in Figure 15.

Surely, Medetera recognizes the silhouette of another Medetera, otherwise Medetera is almost invisible on the bark even for more sharp eyes than Medetera's far vision. To prove that I attached small paper "pseudomedetera" to tree surface (fig. 16) and this pseudomedetera was the object of a full courtship program with wing helicoptering and stroking. I also observed natural misrecognizing: a male courting a tree bark tooth once, another time courting a small Medetera pallipes. So, male mating strategy may be formulated as follows: "I'll do my best and I don't much care whether the female refuses mating with me because she is not ready to or because she is a male or because it isn't fly at all."
But it still does not explain why the fly in the upper position is male in 8 cases out of 10. I have two guesses. First guess is that males have to do their best to find more silhouettes of other Medetera on the trunk. It does not matter what exactly is being done for best searching (for example, to choose higher points for observation or to change points more often), but searching male itself becomes a better visible silhouette for other males which immediately come to try to mate with him. The second guess is connected with the first one: mating attempt catalyzes reciprocal mating attempt. I observed a comical scene. Male N1 sits on a bark, Male N2 jumps to him in the lower position and begins to stroke him. Being a male with traditional sexual orientation, Male N1 finally jumps out. But after that, Male N1 can see the silhouette of another Medetera (it is Male N2). Male N1 jumps to him in lower position and begin to stroke Male N2...
Medetera, these raptors of tree bark, are peaceful towards each other and do not show any territorial behavior.
Ovipositing habits are a mystery to me, but I think that a negative result has to be reported as well. I have often seen Medetera females with a fatty abdomen, especially at the end of August and September. Fig. 17 shows Medetera on 5x5mm paper and yellow eggs which I squeezed out of female's abdomen. The eggs are large enough that I hardly imagine I can overlook ovipositing I have been especially looking for.

Besides large eggs there is another thing to consider. Medetera female has long ovipositor, and it is rather natural to suppose that it uses this ovipositor for ovipositing. I have observed Medetera with pulled out ovipositor 3 times. Female pull ovipositor out, massaging their abdomen by hind legs (fig. 18). If female stop this massage, ovipositor begins spontaneously pull in abdomen (and it seems to me that female has not possibility to pull it out without help of legs). An undisturbed female repeated this procedure 2-5 times. Still eggs-laying has never been recorded by me.

Medetera pallipes
Medetera pallipes is the second species I found on chosen trees. This much smaller and more timid species is more difficult to observe. The scant information I have is:
1. Medetera pallipes avoids a height of less than 0,5 m;
2. Medetera pallipes also use wing stroking as courtship (fig. 19) (**see also below).

* Dolichopodidae feeding strategies
In total I have observed feeding in 5 different genera of Dolichopodidae. I have doubts that the common view of Dolichopodidae as predators which suck out their prey is really as common in Dolichopodidae.
Campsicnemus scambus was observed twice as a predator that really sucks out its prey. The prey (Collembola, Entomobryidae, Lepidocyrtinae) was large, sucking took 10 min and afterwards the empty skin of prey was discarded (fig. 20). Once it made an unsuccessful attempt to attack a large chironomid on the water surface (fig. 21).

Hydrophorus praecox was observed sucking out a large chironomid (fig. 22).
Hydrophorus borealis was observed 7 times swallowing Oligochaeta (freshwater worms) (fig. 23-1). It is amazing that for catching worms Hydrophorus borealis put its head completely under water surface!
Hydrophorus borealis detected numerous and helpless aphids on silt, but never attacked them (fig. 23-2).

Liancalus virens was observed once swallowing some worm-shaped creatures (most probably Diptera larvae) in very same manner as Hydrophorus borealis.
Dolichopus sp.(1) was observed swallowing tiny Collembola (Sminthurides penicillifer (?), Sminthurididae) along with a good amount of water and silt (fig. 24).

Recent observations of another Dolichopus sp.(2) convince me that it acts exactly as hen walking in the yard and pecking invisible small objects (fig. 25). During such feeding a large (6-7 mm) Dolichopus sp. female did not pay any attention to the adult Collembola on the same leaf.

Thus, so far Campsicnemus scambus and Hydrophorus praecox appear really to use "suck strategy", but Medetera jacula, Liancalus virens, Hydrophorus borealis, Dolichopus sp.(1) and Dolichopus sp.(2) appear to use "swallow strategy".

** Medetera truncorum
During the period 20 -15 October, 2006 I observed the Medetera truncorum on an old Platanus orientalis and stone fence around it on city park of Adler, Sochi region, Russia, for a total of 4 hours.
1. All main characters of behavior of Medetera jacula are true for M. truncorum: waiting and hunting positions; cleaning; hunting habits; swallowing of prey; courtship, including male to male (fig. 26-2); lack of ovipositing observations.
2. Wing helicoptering looks rather different: wings during helicoptering almost lay on abdomen.
3. Liquid excrements were always whitish as shows Figure 26-1 (M. jucula ? always blackish).
4. Short list consisting of 5 preys is:
Collembola, Symphypleona1
Thysanoptera2 (fig. 26-3)
Diptera, Cecidomyiidae?1
Coleoptera, larvae1

I am very grateful to Dr. Tony Irwin for his kindness to check the text (both as Dipterologist and English), his grate help in prey identification and a lot of priceless advises. I thank Dr. Igor Grichanov for his identifications of Medetera species.