A dagger fly: Empis tessellata !

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In a previous post I introduced you a family of daunting predatory flies with exceptional aerial skills: the Asilidae. I would now present you another family of predatory flies also with interesting habits: the Empididae. These diptera are usually small, except for a few species that can easily exceed 10 mm. A number of these large species have a large and thick “trunk” (their proboscis) that earns them the English name of “dagger flies”. Many adult Empididae feed on other insects, but some species feed on pollen and nectar.

Where it gets interesting, as often, is when you look at the sexual mores of these flies. To attract female favours, males will give them a nuptial gift, taking the form of a prey wrapped in silk! The basitarsi (tarsus segment linked to the tibia) of males’ front legs have special cells allowing them to produce silk. Sometimes the gift is even empty! It’s not without reminding of the behaviour of Pisaura mirabilis males that I introduced you recently! But there is even stranger! The general rule for insects and spiders is that it’s males who develop ornaments (which can take the form of bright colours, long setae, inflatable segments…) to increase their attractiveness to females. The main reason for this seems relatively simple: developing these ornaments is a costly energy process. Female gametes are expensive to produce, far more expensive than male gametes, and fewer in number. Also, if females had to spend energy to develop ornaments like males do, it would be at the expense of producing their gametes and therefore their fertility. However, in the Empididae, it seems that we are seeing exactly the opposite: females develop such ornaments, sometimes excessively, rather than males! And we can also see large competition between females, whereas it is usually a male-male competition for the other insects.

Several assumptions were made to try to explain this apparent reversal of roles, but no consensus emerged. A recent article from 2020 seems to have found a correlation between female ornamentations and… male testicle size. Females with more ornamentation are more likely to copulate with several males. When a male copulates with a female, it “wants” its sperm to fertilize female gametes. Several males copulating with the same female implies competition between the sperms of the different males. If a male has greater testicles than his brothers, that implies he can ejaculate more sperm than the other males, and thus increases its chances of seeing his sperm win the fertilization race.

I’m not a great connoisseur of this family in terms of specimen identification, and my individual (found in the middle of the pavement, I even nearly walked on it) kindly agreed to lay motionless under my stereomicroscope. He was really placid, not like the Asilidae which are very reactive. So I used an ID key and investigated the various elements to observe under stereomicroscope to get the genus and species. The last two photos illustrate the criteria I used: the presence of a discal cell (cell and marker 1 Disc), the vein r4+ 5 (green line) that splits at its end, the anal cell (cell and mark Ana) which is as long as second basal cell (cell and bmark 2 Bas), the long proboscis (light blue arrow) and setae on metapleuron (white arrows) brought me to genus Empis. Then the thorax with the black lines superimposing on the rows of bristles, the two veins of the disc cell that reach the edge of the wing (2 purple circles), the presence of more than 4 scutellum bristles (I counted at least 8 setae, 6 are indicated by red traits), the large size and many leg thorns (dark green arrows) brought me to the species Empis tesselata. The green line represents the “supposed” edge of the wing with pronounced excision (which I could discern on my specimen under stereomicroscope). This species is easy to identify for people who are comfortable with this family, who don’t necessarily need to go through the careful and thorough exam of all these details like I did. However, in general, identifying an insect requires examining a long list of such details, and / or directly through the study of genitalia (sexual organs).

Last update: May 10, 2021

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