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74 BRITISH BLOOD-SUCKING FLIES BRACHYCERA—TABANIDAE 75
i.e. the smallest specimens, hibernated twice, and produced adults in June
1921.
" It mav be concluded from what has been said that the larval stage lasts
for about a year, but that in the aquatic species it frequently extends to two
years. Under exceptional conditions Tabanidae have been bred from larvae
living in drv places, and here, as a general rule development appears to have
been rapid."
Horse-fly pupae, which resemble those of Lepidoptera, remain stationary
in mud, damp rubbish at the edge of water, or just below the surface of the soil.
Though at lirst generally pale yellowish or greenish, they become darker
(yellowish or reddish brown) with age. Each of the first six abdominal segments
bears in front of its hind margin a circlet of stiff, bristle-like hairs, which
in the genus Tabaniis are interspersed with short pointed tubercules. The tip
of the abdomen is armed with a cluster of six large, tooth-like spines.
The pupal stage lasts but a short time (ten to twenty-three days in Surcouf's
experience"', and the perfect insect makes its escape through a split in the median
dorsal line of the head of the pupa-case. " The hatching of the adult," writes
Surcouf (1924, p. 61^, " would appear to take place at different times of the day
according to the season and the species. On July 22nd, 1908, we saw a T.
?iigrifacies Gobert hatch from sea-sand at 7.30 a.m., while emergences of T.
bromius, Haematopota pliivialis, and Chrysops caecntiens occurred both in the
evening and at davbreak." Of the duration of adult life the same author
remarks (1924, pp. 61-62^ : " Under natural conditions Horse-flies disappear
verv quicklv ; we have often found that Tabanids persisted for scarcely more
than a week at a given spot, and that others then hatched out in their place."
Air. Goffe (in correspondence) makes the interesting suggestion that some
of the "forms " which he has been able to recognise in many British species
of Tabanidae may be biological races correlated with differences in the media
chosen for oviposition. " A quite small form which appears structurally to be
only bromius, but looks quite different, occurs constantly at several localities
where it appears that the larvae have had to feed in loose soil in woods, away
from moisture in any other form."
NATURAL ENEMIES
Tabanidae are subject to the attacks of insect enemies in both the egg and
the adult stage. In Illinois, U.S.A., a tiny parasitic Hymenopteron (Phanurus
tabanivorus Ashmead;. has been bred from the egg-masses of Tabamis atrattis
Fabr, one of the largest and commonest of North American Horse-flies, and in
Austria an allied species {Phanurus (Telenomus) tabani Mayr.) was reared by
Brauer from the eggs of an undetermined species of Tabanus. Other examples
of attacks upon Horse-fly eggs by minute parasitic Hymenoptera have been
recorded.
The adults are sometimes carried off by Robber-flies (Aszlidae). Thus at
Brockenhurst in the New Forest on July 14th, 1894, Colonel Yerbury took a
female oi Machimus atricapillus Fall, feeding upon a male of Chrysops caecutiens
Linn., and Hobby (1931, 1933) in addition to this example, lists captures of
Haonatopota pluvialis Linn, by two species of Asilidae, Asilus crabronijorrnis
Linn, and Machinms at7-icapillus Fall, and a capture of Tabanus bromius
Linn, by Neoitamus cyanurus Loew. The same author records a capture of
Chrysops sepulcralis Fabr. by the Yellow Dung-fly, Scatophaga stercoraria
Linn., observed at Hengistbury Head, Hants by E. R. Goffe.
Spooner (1934, p. 51) found one Tabanus moyitanus Meig. and several
Haematopota phivialis Linn, among the prey rejected by a wasp Metacrabro
quadrici7ictus Fabr. during the reconstruction of its burrow, at Plympton,
Dartmoor in July 1934. Isolated records of Tabanidae as prey of fossorial
wasps are given by Hamm and Richards (1930, pp. 99, 117), and by Hobby
(1932, p. 79). In the United States of America and elsewhere, as reported by
Hine and other observers, wasps, both fossorial species of the family Bembecidae
and also members of the genus Vespa, often play a useful part by pouncing
upon and flying away with Horse-flies which are molesting cattle, horses or
human beings. Hobby (1936, p. 102) quotes a record supplied by Dr. O. W.
Richards of the Dragonfly Cordulegaster bolto7iii Don. preying upon Tabanus
bro}7iius L. at Slioch, Ross-shire, a very unusual locality for this species (see
below, p. 109), but Dr. Hobby tells me that this record has subsequently been
found to refer to T. montanus.
In July 1938, at Laugharne, Carmarthenshire, Mr. J. F. Thomas took five
males and one female of H. pluvialis among the food of nestling swallows.
Mr. Thomas has kindly supplied notes on his previous records. Single specimens
of H. pluvialis were taken also in 1935 and 1937, and of H. italica in
(?) 1933 and 1934. These isolated records suggest that the capture of Tabanidae
by swallows is purely fortuitous.
TABANIDAE AS CARRIERS OF DISEASE
Although the bite of a Horse-fly always involves some risk of accidental
infection of the wound, and may set up considerable irritation in susceptible
persons, no Tabanid in this country has ever been shown to transmit disease,
either in man or in animals. There are, however, a number of exotic species,
especially in the tropics, which are habitual carriers of pathogenic organisms.
The few affecting man all belong to the genus Chrysops : in \A'est Africa C.
dimidiata AA'ulp and C. silacea Austen are responsible for conveying from man
to man the parasitic worm Loa loa Guyot, which is the cause of Calabar Swelling,
and in North America another species, C. discalis Will, is a mechanical transmitter
from jack rabbits to man of the bacterium producing the disease known
as Tularaemia.
Throughout the Old-World tropics occurs a disease of domestic animals
known in India as " surra," and in Egypt as " El-Debab." This is due to
the presence of a flagellate, Trypanosoma evansi Steel, which is believed to be
carried by a number of different species of Tabanidae. There is no evidence
of the development of the parasite within the fly, such as occurs in the trypanosomes
of sleeping-sickness and " Nagana," which are carried by the Tse-tse fly,