DISTRIBUTION OF THE DIPTERA. 78 



retained and nourished in the egg-passages until they are ready to pass into the pupa state, and are 

 born in this condition. 



As regards the internal structure of the Diptera, it may be mentioned that, like the Lepidoptera, 

 they possess a sucking stomach which originates from one side of the oesophagus ; that they have four, 

 or sometimes five, Malpighian vessels, which not unfrequently unite to form one or two ducts by which 

 their secretion is discharged into the intestine ; that the two main trachean stems exhibit bladder-like 

 dilatations, which are often of considerable size in the base of the abdomen ; and that the central 

 nervous chain varies greatly in structure in accordance with the general form of the body, the 

 elongated species having three distinct thoracic and five or six abdominal ganglia, while in some stout- 

 bodied flies the whole of the ganglia seem to be united into a single mass. 



The Diptera are for the most part active day-flying insects, like the Hymenoptera, which some 

 of them closely resemble in appearance and in haunting flowers, upon the sweet juices of which they 

 feed. There is a further resemblance between the Diptera and the Hymenoptera in the prevalence 

 of the habit of parasitism in certain groups of both ; and in both orders we find species whose larvse 

 reside and feed in the interior of excrescences produced upon plants by the irritation of their presence. 

 In the Hymenoptera, however, we have no examples of the bloodthirsty propensities which charac- 

 terise so many Diptera. Apart from these and some other forms which are either annoying to 

 ourselves or injurious to our property, the Diptera must be regarded as acting a beneficent part in 

 the economy of nature. Thus, so far as we are concerned, the parasitic and some predaceous species 

 act as a check upon the multiplication of many other insects which are enemies to the gardener and 

 agriculturist ; while a great number of others seem to have a special mission to clear away all sorts oi 

 decomposing animal and vegetable matters which, if left, would seriously contaminate the air. -i 



From the imperfect knowledge that we possess of the exotic species of this order, it is not an 

 easy matter to arrive at any trustworthy estimate of the total number of species included in it. Dr. 

 Schiner, of Vienna, estimates the known European species at about 9,000, and these he regards as 

 not more than a twentieth part of the whole. Allowing for probable exaggeration in the estimate of 

 European forms, this would give 150,000 or 160,000 as the total Dipterous population of the globe. 



As regards their geographical distribution, the Diptera show a remarkable uniformity, the 

 principal families being generally represented in most regions, and many genera, and even species, 

 having an exceedingly wide distribution. The largest and finest species in this, as in other orders, 

 are generally inhabitants of warm climates. In their geological history, the Diptera agree generally 

 with the Hymenoptera. No Palaeozoic Diptera have been recorded. Among the Secondary rocks 

 the Rhaetics are the earliest in which remains of Diptera are supposed to occur, and these arc 

 regarded as very doubtful. The Lias of Schambelen in Switzerland, which has yielded Professor 

 Heer such an important assemblage of insects of various orders, contains no Diptera, and it is 

 not until near the close of the Mesozoic period that undoubted remains of Diptera were preserved 

 in the lithographic slates of Solenhofen and in the Purbecks of Dorsetshire and of Wiltshire and 

 Buckinghamshire. These represent several still existing families. A few fragments of wings occur 

 in the Wealden ; but in all Tertiary deposits Diptera occur in constantly increasing number and 

 variety, but all referable to families and usually to genera still living on the earth. 



The Fleas (Ap/ianiptera) have of late years commonly been regarded as aberrant members of 

 the order Diptera, but for various reasons we have preferred here to revert to the old view, never 

 entirely abandoned by nattiralists, that these insects form a distinct order. The remainder, 

 characterised in general as above described, may be divided into two groups, in accordance 

 with a very important difference in their mode of reproduction. In the first and most typical 

 group, the females produce their young either in the egg state or as young larvae ; in the second, 

 the hatched young are retained within the oviduct until they are full grown, and only extruded 

 when they are just about to pass into the pupa stage. These latter, to which the name 

 of PUPIPARA has been given, differ from the rest of the order in several points of structure; 

 thus, the head is either very closely applied to the front of the thorax, or actually immersed in a 

 cavity formed in that part for its reception, the antennae are excessively short, and usually immersed 

 in cavities of the front of the head, and the maxillae form a sort of sheath for the labrum, the 

 actual piercing organ consisting only of these three bristles. The remainder, which exhibit, with 

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