BRITISH 
BLOOD-SUCKING FLIES. 
IN the shape of the common house-fly, or the blue-bottle, Flies 
are familiar to everyone, and a brief examination of either of 
these household pests will reveal two of the chief characteristics of 
the Order (DlPTERA) to w hich they belong, — the possession of but 
a single pair of wings, and, immediately behind these, the presence of 
a pair of little knobbed organs, the halteres or balancers, which 
represent the second pair of wings possessed by other insects. These 
two features, — the single pair of wings and the halteres, both of which 
can clearly be seen in the majority of the plates illustrating the 
present work, — serve to distinguish all ordinary Diptera from all other 
insects. The winged males of Coccida; (Scale-insects), which belong 
to the Order Rhynchota, though they have only one pair of wings, 
and might perhaps be mistaken for gall-midges (Diptera), are 
distinguished by the possession of a pair of long caudal filaments at 
the tip of the abdomen, and by being without halteres. In a small 
number of aberrant Diptera, as in the sheep " tick " (Plate 34), the 
wings, or both wings and halteres, are entirely wanting, but in these 
cases the other details of the insect's external anatomy disclose its 
systematic position. Under the term " Flies " we include then, not 
only the horse-flies (Tabanidse) and many other families, the species 
of which more or less resemble the house-fly in shape, but also the 
midges and mosquitoes, which, though very dissimilar from the 
former in appearance, nevertheless possess all the essential structural 
characters of Diptera. 
Excluding the Fleas (Pulicida:), which it is better to regard as 
forming a separate Order of insects, 59 families are recognised in 
Verrall's 'List of British Diptera,' 2nd Edition, (Cambridge, 1901). 
Of these, if we leave out of the question the highly specialised and 
