52 University of California Publications. [Ejjtojiology 



peculiarities from those of other similar invaginations, as for 

 instance, those of the legs. The wing disks, as a whole, are 

 from the first different in shape from other invaginations, and 

 thus distinguishable from the leg disks; later the arrangement 

 of the different sorts of cells that compose them is characteristic, 

 and ultimately they are distinguishable by the structure itself 

 of some of the cells; but this is not true of the wing funda- 

 ments when first developed. 



All wing fundaments are alike to the extent that they are 

 sac-like organs and that the mouth of the sac opens into the 

 general body cavity. Being thus open the sacs always contain 

 blood and are accessible to ingrowths of trachese and connect- 

 ive tissue; the latter forms in many cases an investment of 

 the deep ends of the cells which constitute the wing mem- 

 branes, and is generally conspicuous when the wing is young. 

 This investment has been noticed by several authors in wideh' 

 different groups, and may occur in all cases. Ultimately this 

 connective tissue may partially or wholly disappear, or may in 

 other cases increase in amount and contribute not a little to 

 the stiffening of the wing. An extreme case of this kind may 

 be seen in the front wing of Belostoma, where it extends as a 

 strong layer across the whole breadth of the outer third of 

 the wing and is thickened at the vein so that if the whole 

 cuticle is stripped off (after loosening it in caustic potash) a 

 complete venation and membrane still remain. In this case 

 the cuticle over the vein differs from that elsewhere only 

 slightly if at all, so that it would be difficult to make out the 

 venation from the cuticle if it were taken off and examined 

 alone. The chitinization of the connective tissue here is more 

 complete than that of the cuticle itself, so that it is harder and 

 more resistant to the action of alkalies. 



The presence of nerves associated with tactile hairs or other 

 sense organs has often ])een noted in company Avith the veins. 

 They need not be treated of in this place, as they are not uni- 

 formly present in veins, and certainly are not a necessary or 

 important component of them. 



The problematical "rods of Semper," that have been 

 repeatedly described in Lepidoptera, are only imaginal tracheje 

 in the process of development and not essentially different 

 from any other trachea^ in insects' wings. 



In studying the development of veins we need only to take 



