2 AN ACCOUNT OF BRITISH FLIES. 
Order DIPTERA. 
Mouth suctorial, and used for piercing, the labium being greatly 
developed and forming a gutter for the reception of the lancets 
{maxillee and mandibles). 
This proboscis, which is chiefly made up of the labium, ends in a 
fleshy swollen tongue, and is without palps. It is closed above by 
the labrum. 
The maxille are provided with palps; the- mouth, however, may 
be obsolete or closed. 
The head is usually large and spherical, and is attached to the 
thorax by a short neck. Two large facetted eyes are present, and 
there may be ocelli as well. In the ¢ the eyes may join in the 
middle line, this being a valuable way to distinguish sex. 
The antenne are of two forms, («) long and filiform, and composed 
of many joints ; or (8) short, with the third joint inflated, and bearing 
two or three tapering joints or a bristle. 
The thorax is usually large and compact ; the prothorax very much 
reduced, and attached as a collar to the mesothorax, which is very 
large and is developed at the expense of the other two regions of the 
thorax. 
One pair of true wings only developed, these being the anterior 
ones, and are usually transparent and never folded, with an inferior 
basal lobe. Veins crowded in front, distant behind. Cross veins 
few. 
The hind wings reduced to small club-shaped laminz, known as 
halteres. Both may be absent. The abdomen generally small, but 
may be elongated, as in the Tipulidz, and composed of five to nine 
segments. 
Legs six in number, and slender, having five jointed tarsuses, 
ending in ungues or “‘pads” for attachment. The nervous system 
presents various modifications—in some the ganglia of the thorax 
and abdomen become fused, or there may be three thoracic ganglia 
and five or six abdominal ganglia. The male genital organs consist 
of two testes with vasa deferentia and copulatory appendages. 
The female have three receptacula seminis in connection with the 
vagina, and in some cases have a retractile ovipositor. 
Having given a short general description of the Diptera we can 
now pass on to the review of the fossil species. In dealing with the 
ancestral forms of our flies, we include those found in all parts of 
the world, as very few have been discovered in England, compared 
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