404 NATURAL HISTORY OF ARTHROPODS. 



transparent, though sometimes prettily spotted or more or less colored. They are 

 naked or clothed with microscopic hair, and supported by a system of longitudinal and 

 transverse veins, which are always present though varying much in number and intri- 

 cacy ; in some species, as the Tipulidae and Nemistrinidse, almost Neuropter-like in 

 their reticulation, while in others, as especially the Cecidomyidae, there may be only 

 three or four weak longitudinal veins present. Some of the most important family 

 characters are drawn from the number and arrangement of the veins, especially on the 

 basal part of the wing, while the outer portion may furnish very valuable generic 

 characters. 



The head has a very free union with the thorax by means of the slender neck. 

 It varies exceedingly in shape, and, as would be supposed, furnishes many of the char- 

 acters for classification. It is more usually hemispherical, with the occiput flattened 

 or even concave, but often is spherical or conical and sometimes with lateral prolonga- 

 tions upon which the compound eyes are situated. 



The eyes are almost always well developed, comprising a large part of the head or 

 indeed often much the largest part ; they are composed of a great many separate lenses 

 or facets, which in not a few are enlarged in areas in the male and are often with bril- 

 liant markings. As a general rule the eyes in the male approach each other, or are 

 contiguous, above, leaving only a small space at the vertex and another below near the 

 antennae. At the vertex there are generally three simple eyes, or ocelli ; their presence, 

 however, is inconstant in closely allied forms sometimes, and cannot hence be of 

 important use to such. 



The antennas vary much in structure and are very useful, both in distinguishing the 

 higher and lower groups. They are usually situated near the middle of the head in 

 profile, though they may be placed above near the vertex, or below near the mouth. 

 Until recently, and yet by many entomologists, all Diptera were divided into two sub- 

 orders, according to the structure of the antennas. In those of the first division, the 

 Nemocera, the antenna? are thread-like, consisting of from six to thirty-six joints, all 

 of which, except the first two, being alike in structure, and often with a circle of 

 hairs on each joint. Of this group the mosquito will readily serve as an example. 

 Under the Brachycera were placed those families among which the antennas consist 

 only of three joints, and the additional ones, whether distinct or styliform, or most 

 frequently bristle-like, are considered only appendages of the third joint. In the most 

 typical forms, such as the housefly, the antenna? are qtiite short, composed of three 

 simple joints, the last of which has a slender bristle on its upper border. This bristle, 

 which frequently is ornamented feathery-like, is more or less distinctly jointed, and in 

 reality corresponds to the additional joints of the first sub-order. It is true it may be 

 situated close to the base of the elongated joint, but this is due to the fact that this 

 joint is chiefly or almost wholly developed from below. Although these two general 

 divisions may seem useful enough when the more typical forms alone are observed, yet 

 intermediate forms are such as to render the division unnatural and indefinite. At 

 present a more natui'al division is that proposed by Brauer, based chiefly upon the 

 study of the metamorphoses in the immature stages, and which will be defined 

 further on. 



The mouth parts of Diptera are wholly suctorial, and differ from those of Lepidop- 

 tera in that all of the component parts may be brought into use. They differ not a 

 little, however, in different flies, as might be supposed from their diverse habits. In 

 some they are adapted for piercing animal or vegetable substances, and are, in con- 



