Adult gasterophilines are bee-like flies with hairy head and thorax but few bristles. The mature female has the abdomen extended as an ovipositor.The adult flies are highly variable with respect to the body colouring and pilosity. The typical form has the thorax densely beset with predominantly yellow-brown hairs dorsally, and whitish ones laterally around the humeral calli and near the scutellum. The abdomen is tricoloured, bearing long white hairs on the second segment, predominantly black ones on the third and fourth segments, and dark yellow hairs on the posterior parts.
The adults are diurnal with peak activity occurring in the early afternoon in warm, sunny weather. Adults are short-lived and, given favourable conditions, mature ovipositing females have an effective field life span of one day. They do not feed and the mouthparts are greatly reduced. Mating occurs in the vicinity of horses when solitary hovering males will establish and defend a territory. Mating also occurs on hilltops where the males hover and aggressively pursue passing objects, presumably in search of females. Males make contact with a female on the wing, couple, and sink to the ground where copulation is completed in 3 to 4 minutes.
In the horse stomach the red larvae of Gasterophilus intestinalis favour the cardiac region and cluster at boundary of glandular and non-glandular epithelium whereas the yellowish Gasterophilus nasalis larvae attach around the pylorus and sometimes the duodenum.The third instar larvae may remain attached by their mouth hooks for up 12 months.
Mature larvae of Gasterophilus are large and stout and there are bands of large spines on all segments except the last. The characteristics of such spines can be used to identify species.The third instar of Gasterophilus intestinalis is red and has two rows of coarse spines per segment that are blunted at their tips. It is usually found in clusters in the nonglandular part of the stomach either near the margo plicatus or in the saccus cecus.
Deep brown, with the features of the third instar larva.
The larvae of Gasterophilus sp are parasites of equines and are known as bots. The Gasterophilus larvae become parasitic when licked by the host, thus the infestation starts in the mouth. The larvae eventually migrate to the stomach where they attach to the inner lining with their bodies exposed to the stomach cavity. When mature they exit in the faeces, then pupate.Adult fly live for a few days whilst mating and laying eggs.
Five days after beig laid, the eggs of Gasterophilus intestinalis contain first instar larvae that are prepared to hatch rapidly in response to the sudden rise in ambient temperature that occurs when the horse brings its warm muzzle and breath in contact with them; they do not respond to gradual warming. In the mouth the larvae spend up to a month and develop to the 2nd instar. The 2nd and 3rd instars are attached to the wall of the stomach.
Ovipositing females will lay eggs on walking and trotting horses in addition to standing horses. If this action induces the horse to gallop, the females will pursue galloping horses alongside the flank until the host sops when the flies will immediately resume oviposition. This urgency to oviposit reflects the short life span of the adults.
The eggs of Gasterophilus intestinalis on the hairs of front legs are far removed from their destination and depend on direct assistance from the horse to find their way into the mouth. About 87% of the total bot fly eggs are laid on hos innert foreleg and knee areas.Thus, eggs are placed in an optimal location for ingestion, and the physical presence of the eggs may stimulate grooming.