408 NATURAL HISTORY OF ARTHROPODS. 



head, usually without eyes, more or less worm-like in appearance. Some resemble 

 snails somewhat, and may construct for themselves delicate silky cocoons ; a few 

 species form galls. The wings of the adult fly have but few veins and no discal cell, 

 which together with the elongate coxae will render these flies easily distinguishable. 

 Usually there are ocelli present, and the tibiae are all armed with spurs. The flies are 

 small, active usually, leaping about by the aid of their hind-legs. 



The larvse of one genus, at least (Sciara), have long been known for their grega- 

 rious habits. They are often found in dense patches under the bark of trees, and, what 

 is more interesting, when about to change to the pupa state, will congregate in im- 

 mense numbers, forming processions that have been observed four or five inches wide 

 and ten or twelve feet long. They travel in a solid column from four to six deep, over 

 each other, advancing about an inch a minute. From this peculiar habit, they have 

 been called the army-worm in Europe. Similar habits have been observed in this coun- 

 try among our species. One species of this genus {S. onali) is known to feed in num- 

 bers in the interior of apples in this country. 



The species of Mycetohia figured was seen in abundance by Packard "in the 



crevices of the bark of an elm from which flowed 

 a sour sap mingled with dust, and in this putres- 

 cent mass the slender white larvae were seen 

 gliding about." 



Among all the flies that constitute this 

 group, there are perhaps none so injurious as 

 the Cecidomyidje, or gall-flies. The family 

 Fig. 511.— Afi/ce«o6iasorrfjrfo,- a, larva; Contains a large number of extremely delicate 



, pupa en arge . ^^^ vcry minute species, clothed on the wings 



and other parts of the body with long hairs, which are easily rubbed off. There are 

 no ocelli, the thorax is without a transverse suture, the coxse are not elongate, the 

 femora not thickened, and the tibiae are without spurs. The wings, moreover, contain 

 but very few veins, there being only three or four longitudinal ones apparent. 



The family must embrace a very large number of species, but owing to the unavoid- 

 able difficulties that their study must always present to the pure systematist, a thorough 

 knowledge of them will be obtained very slowly. At present less than six hundred 

 are known. The knowledge of our American species is chiefly due to Baron Osten 

 Sacken, to whom, more than any one else, the science of American dipterology is owing. 

 The following account of the larva is taken largely from his writings. 



The egg of Cecidomyia is elongated, rounded at both ends, orange yellow or 

 whitish. The time when the larva is hatched ^•aries much, and depends upon the 

 state of the weather, sometimes requiring only a few hours, but more generally a few 

 days. When fii-st hatched the larva is colorless, transparent, with a translucent green, 

 yellowish or red stomach ; late in life it assumes different shades of red, or becomes 

 yellow, or whitish. All these larvae have the extraordinary number of fourteen seg- 

 ments, thus affording an apparent exception from all the lai-vae of insects, which as 

 a general rule have thirteen. This supernumerary segment is placed between the head 

 and the first thoracic segment. The larvae have nine pairs of stigmata along the side 

 of the body, apparent as long, more or less nipple-shaped projections. The head is not 

 differentiated ; the jaws are rudimentary, and there are large two-jointed palpi. On 

 the second segment above there is a pigmentary eye spot. Below, on the same seg- 

 ment, there is, according to the age and species of the larvae, a more or less developed 



