56 INSECTS. 



shown at b in the figure on p. 55. This stage lasts from eight to twelve days ; 



but the perfect insect is shortlived, the female surviving apparently only long 



enough to pair with the male and lay her eggs. 



When speaking of one of the true midges reference was made to a pathological 



case of phosphorescence, but in the present family there are two instances known 



of the normal occurrence of this phenomenon — not, however, in the adult insect, but 



in the larval or pupal stages. The first instance is furnished by Ceroplatus sesioides, 



a midge, which although not yet known to occur in England, has been met with in 



several of the countries of Europe. Here the luminosity is said to resemble that of 



the glow-worm, but proceeds from the entire animal, and from members of both 



sexes. The larvae, which are found in small colonies on the under side of a fungus, 



exhibit, when crawling in the dark, a moving streak of light, less bright than that 



emitted by the pupae. The insect also shines when lying in the cocoon, so long as 



its abdominal rings are still transparent and have not attained their complete 



colouring. The cocoons themselves are not luminous, but allow the light to be 



transmitted as through a paper lantern ; and since as a rule several of them are 



situated together a more extensive glow is displayed, whereby both the cocoons 



themselves and the surrounding objects are illuminated. When the insect is about 



to emerge from the cocoon, the luminosity gradually diminishes, and ultimately 



ceases altogether. The second instance is presented by a New Zealand midge 



called Boletophila luminosa, the larva of which is known as the " glow-worm." 



Here the female is luminous in all three stages of its existence, but in the male 



the luminosity disappears two or three days before the emergence of the perfect 



insect. The luminous organ, which is situated in the posterior part of the body of 



the larva, consists of a gelatinous, semi-transparent structure, capable of extension, 



contraction, and other changes of form, and, like its luminosity, is completely under 



the animal's control. As to the part played by this organ in the midge's economy, 



authors are at variance ; one believing that the light serves to attract small 



creatures, so that they become entangled in a web of mucus, which the larva 



suspends in some niche in the soil. 



The gall-midges (Cecidomyidce) are minute, fragile insects, in 

 Gall-Midges. . te . „. . . p i • 



which the wings are furnished with few veins, are often hairy, and 



always fringed on the edges. From an agricultural point of view, these insects are 



the most important of all the gnat-like flies, since much damage is at times done to 



crops by their larvae. The most notorious is the Hessian fly (Cecidomyia destructor), 



represented in its various stages in the accompanying illustration. This insect was 



believed to have been introduced into North America by the Hessian troops at the 



time of the War of Independence, whence the inhabitants of the United States 



gave it the name by which it is now commonly known. The adult female, which 



measures rather less than a tenth of an inch, is mostly of a velvety black colour, 



varied with blood-red, especially on the abdomen ; while the rather larger male is 



browner, with the red clearer. These flies may be observed on the wing during 



the second half of April. They live, however, only for a few days, and perish soon 



after laying their eggs, which amount to about eighty or a hundred. These are 



placed separately or in pairs upon the leaves of the wheat-plant, and in a short time 



hatch, when the larvae crawl down the leaf, reach the stalk, and burrow in it to take 



