It is associated with a wide variety of waterbodies, from large lakes and rivers down to areas as small as ditches, small ponds or even muddy puddles. Larvae have even been found in cow-dung, very wet manure or very wet old sawdust.
This species visits flowers; it also commonly rests of leaves. It often emits a buzzing sound when resting. It is a notable wanderer and can be found well away from water.
The larvae feed on detritus.
Like other members of the genus Helophilus it has black and yellow longitudinal stripes on the upper surface of its thorax. The abdomen is patterned with yellow, black and grey. It has a black central face-stripe. It has a wing-length of 8.5 - 11.25 mm.
The second and third tergites are yellow patterned with black. The black pattern consists of a band across the front of the tergite (in its central part only, not extending to the tergite's edge), and stripe down the centre of the tergite, and a black band across the full width of the rear of the tergite. At the very rear of these tergites there is usually a very narrow yellow rim. This rim is usually slightly thicker on the third tergite, where it is also more obvious as the front of tergiter four is wholly black, thus enhancing the contrast of the feature. Several other Helophilus species have this rim, but Helophilus trivittatus does not. Due to the black pattern described above, the main yellow areas on tergites two and three are at the sides of the insect, but they are fairly prominent due to their large size, and their bright colour. The yellow here is usually a bright, slightly orangey yellow (unlike the paler lemon yellow of trivittatus). Tergite four is largely black, with isolated dull yellow crescentic markings on each side. Females have yellow hairs on the hind part of tergite five, unlike H. hybridus females, in which these hairs are black. Male hybridus appear quite different from male pendulus as the black band on the rear of tergite two does not extend to the edge in that species.
The legs possess patterning which differ from the other species in the genus. First, the hind tibia of pendulus has at least the basal half (usually all except the distal third) pale yellow, whereas other species, such as H. hybridus and H. groenlandicus have pale yellow only on the basal third or less. The middle tibia of pendulus is wholly pale, unlike H. groenlandicus, on which it has a dark apex. The mid-tarsi have extensive dark markings on at least their distal segments, unlike trivittatus which usually has wholly pale mid-tarsi.
The larva is a rat-tailed maggot.
- Stubbs, Alan E. and Steven J. Falk (1983), British Hoverflies, an illustrated identification guide, British Entomological and Natural History Society