Syrphid detail

Sometimes I have to hunt for specimens, sometimes specimens come to me - in this case the hoverfly (family Syrphidae) Helophilus pendulus - a common species, often seen in our garden, in this case found recently dead on a windowsill. It could have been swept into the bin, but not before treating it as an opportunity to look at external hoverfly anatomy in the following series of annotated images.

Helophilus pendulus, dorsal view - note the strong, almost semi-circular, loop in wing-vein R4+5 (about a quarter of the way in from the wing-tip), characteristic of the hoverfly Tribe Eristalini. The pattern shows that the species (and genus) is a wasp-mimic. The fainter 'false vein' or vena spuria can be seen running much of the length of the wing (approximately in the middle) and is only found in hoverflies.

Helophilus pendulus, front view - the 'face' is clearly seen with its dark vertical stripe in the centre, running across two protrusions - (1) the 'knob' and (2) the 'lip' above a strong indentation.

Underside of the abdomen showing the hind legs most clearly - the hind tibia is at least half pale in H. pendulus as shown by the red bar.

The black-and-white pattern of the thorax is visible here as well as the fine bristles around the compound eyes (note the row of four relatively large dark bristled pores just behind the centre of the rear margin of the eye) and the ocelli (simple eyes) on the top of the head (red arrow).

Dorsal view. The red arrow indicates the semicircular protrusion ('scutellum') behind the thorax which covers (and presumably protects) the upper rear of the thorax and delicate membranes between that and the abdomen. The short bar top-left indicates the yellow 'gap' between the characteristic black abdominal markings and the rear of the segment.

As in all true flies (Diptera), the 2nd pair of wings is reduced to a small drumstick-like 'haltere' which has a counterbalancing/gyroscopic function beating the opposite way to the wings in flight.

The large posterior spiracle (red arrow) near the base of each wing - used for breathing along with the anterior spiracle found just above the base of each front leg, just behind the head.

One of the 'feet' - the 5th (of 5) tarsal segments is shown by the red bar - there are two claws (solid arrow), each curving past a gripping pad (broken arrow).

At the base of the wing there are various tufts of hairs which help cover the various joints between body plates etc and prevent material getting in which might cause damage or interfere with movement.

The front of the 'face' - the central vertical stripe ends at the 'lip' above a deep indentation.

The antennae are short and consist of three segments (numbered here) - the 3rd is expanded and bears a long sensory bristle called the 'arista'.

The lenses of the compound eye.

The extended mouthparts which form a three-segmented sucking/licking structure including sensory apparatus (e.g. for testing potential food), muscular pumps and so on. Liquid food is taken - dried nectar or pollen must be dissolved/suspended in saliva before it can be consumed.

There is of course more that could be covered here (and I may expand some parts with more detail in future posts), even without dissection, but if you want to know more, the following are books I regularly use:

• Rotheray, G.E. & Gilbert, F. (2011). The Natural History of Hoverflies. Forrest Text, Cardigan. Does what it says on the cover. More morphological detail/images would be useful, but still good coverage of natural history.
• Stubbs, A.E. & Falk, S.J. (2002). British Hoverflies. BENHS, Reading. Keys to adults of British species, many plates, including dissections of genitalia, detailed species accounts, and useful background information.

Enjoy!