FLIES. 409 



hardened, chitinous spot, the so-called ' breast-bone,' which is retractile into a trans- 

 verse cleft, and serves for locomotion. The skin is very finely roughened or smooth, 

 and some have, in addition to the breast-bone, false abdominal legs. The last segment 

 is smooth or rounded, or furnished with two elongate tubercles, sometimes uneven 

 and bristly, or armed with a pair of horny processes, frequently curved upwards, that 

 may serve as aids in leaping, which is effected by pressing these horny hooks against 

 the under side of the thoracic segments. Their motions, except those few which live 

 on the surface of leaves, are generally slow; but those which change their abode 

 before assuming the pupa state become very active about this period. A very great 

 activity was observed by Winnertz in some such larvae after a thunder-storm. They 

 left their hiding places under ground and crawled about restlessly for some time; 

 repeating these actions after every thunder-storm, some of them even two months after 

 having left their galls. 



Owing to their rudimentary jaws it seems evident that the larvae must feed upon 

 juices only, and that they require but little nourishment is shown by the fact that they 

 attain their full growth and development in a gall just large enough to enclose them, 

 apparently hermetically sealed, and for the most part with hard walls. It is most prob- 

 able that they absorb nourishment in a quiescent state. As many as sixty are known 

 to live in a single gall. What causes the galls is not so easy to answer ; probably some 

 peculiar irritation produced by the insect. 



Most of the species live exclusively on a single kind of plant, or at least closely 

 allied plants. The greater number penetrate the inside of the plant so as to be con- 

 cealed from view during their development. Their presence is generally indicated on 

 the outside of the plant by some deformation. Every part of the plant from the flower 

 to the roots is liable to such attacks, but each species attacks the same part of the 

 plant and deforms it in the same manner. 



The larvae are, unlike all the allied families, often pupigerous ; that is, the larval skin 

 contracts to form an envelope for the enclosed pupa. They, however, differ from the 

 cyclorhaphous larvae pupigerte in that the fly does not escape through 

 a circular opening at the anterior end, but the abdomen is protruded 

 through a transverse opening between the seventh and eighth segments, 

 the head escaping last. Some of the larvae form cocoons for them- 

 selves before undergoing their final transformations. They have been 

 observed by Winnertz, when fastened to a leaf, to become encircled in 

 twenty-four hours by a white halo, consisting of tiny, thread-like par- 

 ticles ; which seemed to gi'ow somewhat like crystal particles ; the 

 larvae during the time remaining perfectly motionless. The cocoon 

 was perfected in a few days, but even then, though examined under a 

 a strong magnifying power, no genuine thread was perceptible. 



A most remarkable fact in the biology of these insects was dis- 

 covered by Wagner, in 1860. According to this scientist the larvae 

 of certain species, which he placed under the genus Miastor, and a 

 which live under the bark of trees, produce from ovary-like organs 

 a number of eggs, which hatch within the abdominal cavity of the ^^^id^iS^^^im- 

 parent and here remain awhile, feeding upon the tissues that sur- daand't)Beudova^' 

 round them until they have been consumed; they then escape to 

 increase in size and produce another generation in the same remarkable manner. 

 These series of sexual reproductions are continued from the autumn to the following 



