No. 2.) LARVA OF CTENOPHORA ANGUSTIPENNIS. 555 
IX. THE IMAGINAL Bups. 
The imaginal buds are small whitish bud-like bodies which 
lie between the muscles and the body wall of the thoracic 
segments. There are two pairs of these imaginal buds in each 
segment. There are several present on the ventral surface of 
the live animal when studied with a hand lens. The ventral 
invaginations give rise to the legs, while the dorsal invagina- 
tions develop into the pupal respiratory organ, the wing, and 
the halterer. 
The imaginal buds or histoblasts, which Kellogg suggests 
as a better name for these structures, are composed of an 
invaginated portion of the hypodermis which has become 
folded and in which there has been a special increase of cells. 
(Fig. 42.) During this modification, the outer portion of 
these cells is separated from the rest and forms the very thick 
enveloping membrane, the peripodal membrane. The thick- 
ened part of the histoblast is the portion which later becomes 
functional as the developed organ. This portion forms two 
folds, each fold being composed of several layers of cells. The 
so-called tracheal veins lie between the two layers of the func- 
tional portion of the histoblast. The peripodal membrane and 
the function portion of the bud are both direct outgrowths of 
the hypodermis. 
There are also other structures which, according to Vil- 
lanes, are homologous with the histoblasts; these are the so- 
called optic imaginal buds. (Fig. 43.) The eyeless larva, as 
is common with the other eyeless Dipterous larve, avoids a 
too strong light, and this perception of light through the 
integument is probably due to the presence of these optic histo- 
blasts. If a ray of light is concentrated on the region of these 
optic imaginal buds the response is much quicker than if the 
light be concentrated on another part of the body. The optic 
imaginal buds are rectangular in shape and are like other 
imaginal buds in possessing a peripodal membrane and a wider 
functional portion of the histoderm, which later develops into 
the optic ganglion. In the central cavity there are several 
tracheze and some loose mesodermal tissue. 
