More on Horseflies

Following on from Kim and Neil's record of a female Tabanus autumnalis at Summer Leys, Dave Jackson sent me a picture of another there, but this time a male. In many species of flies, but not all, the males can be distinguished from the females by having holoptic eyes - that is the eyes meet along the top and front of the head. Females are always dichoptic - having a separation between the eyes. Here is Dave's photo:

 Tabanus autumnalis Large Marsh Horsefly (c) Dave Jackson

I mentioned in the previous blog some of the smaller, more common species of horsefly (family Tabanidae) so I have added a couple of examples below. 

Chrysops relictus Twin-lobed Deerfly (c) John Showers

The above horsefly is one of the usual culprits when a nice walk in the country is spoilt by biting flies. They can be very persistant. It tends to like damp places, marshes and damp woodlands are favoured. I was once driven off doing a dragonfly count at Pitsford's Holcot Bay by deerflies attacking me. There are very similar other species that occur locally but in much fewer numbers so I always try to take one or two for closer examination.

Hybomitra bimaculata  Hairy-legged Horsefly (c) John Showers


Notice the eye patterns on the horseflies. They all have coloured eyes and some have distinctive bands or spots. These patterns fade on dead insects so you need to see them live and up-close and personal to appreciate their beauty! These patterns are useful clues to identification so worth noting.

Last Saturday we held our late June Hoverwatch survey at Old Sulehay Forest and Graham Warnes caught another of the medium sized horseflies there Hybomitra bimaculata Hairy-legged Horsefly. These are usually found in woodland rides. I have also found this species at Yardley Chase. Often the males can be seen hovering a few feet off the ground in bright sunshine. They are holding territories to await or attract a passing female.

Summer Leys horsefly

I received an email from Neil Hasdell on monday saying that he and Kim Taylor were walking round the reserve when Kim saw a large insect alight on a fence post. They quickly investigated and Neil realised it was a large horse fly about 20 mm long. He sent me his photos for identification. 

From the photos and size I suspected Tabanus autumnalis but as this is a very scarce species in this area, I sent the pictures to Martin Harvey, who runs the Soldierflies and Allies Recording Scheme. He is happy that it is T. autumnalis, but said that if seen again a specimen should be caught and the underside of the abdomen checked for a dark longitudinal stripe. This is diagnostic. It is only the third record for Northants.

This species is considerably bigger than the more locally common horseflies like T. bromius and Hybomitra bimaculata or the even smaller clegs and deerflies. It is generally the smaller species that are a pest to humans. The larger ones consider us too small! They generally go for cattle and deer. One good way of finding horseflies on hot days is to leave the door of your car open in a sunny area near to where there are cattle. The flies home in on infrared radiation from hot bodies and will enter the car, where you can easily (hopefully!) trap them. Alternatively, if you don't want horseflies in your car, keep the doors shut. At a field meeting of Dipterists Forum at Speyside we parked in a lay-by on the Glen Feshie road and immediately had the giant horsefly T. sudeticus investigating the front wheel arches of the car, attracted by the heat from the engine compartment. Incidently, horseflies tend to be attracted to dark objects, so wearing light coloured clothes when out in suitable habitat may reduce your chances of being bitten.

Tabanus autumnalis (c) Neil Hasdell. Note the eyes do not have any bands across them. The similar, but smaller T. bromius has a single coloured horizontal band. More of Neil's photos below.