of certain species of WiUow. — Part 2nd. 245 



the humeral suture causes these vittae. Again, because iu the typical 

 Gomphus (Pseudoneuroptera) there is a pale vitta, the locus of which 

 is ou the dorsal carina of what is called the dorsum of the thorax, it 

 does not at all follow that the dorsal carina causes this vitta. Lastly, 

 because in the front wing of Noctuidx the loots of the '-orbicular spot" 

 is in the wing-cell above the main stem of the median vein, it would 

 be poor logic to infer that that wing-cell throughout this Lepidopterous 

 family causes the spot. 



It might, as I formerly suggested, ( Proc. etc. V. p. 213,) be assum- 

 ed, that the paleness of the bullae and of the bullar streaks is caused 

 by ;t mere structural thinning out of the wing at these particular 

 points. But an attentive examination of many hundred wings under 

 a high power has satisfied me, so far as one can be satisfied without ac- 

 tually measuring and weighing, that the wing-vein is as thick at 

 the point where the bulla occurs as elsewhere, and that consequently 

 this phenomenon is colorational and not structural, except so far as all 

 color may be caused by difference in the microscopic texture of the 

 surface of the parts. 



"When I discovered these bullae, " says Jurine, the first author who 

 gave any account of them in print, though he entirely overlooked the 

 bulla; F and G, "I presumed that they were apertures through which 

 the air contained in the tracheae [wiug-veins] was forced between the 

 double membrane composing the general surface of the wing. But 

 upon examining them with more attention, and upon reflecting that a 

 great number of Hymenoptera were deprived of them, I abandoned 

 that idea, and considered them as a dilatation of the corneous substance 

 of the tubes, caused by the folds of the wing ; (determines par les pit's 

 de Vaile ;) and in fact it is always in the direction of these folds that 

 the bullae are found."* 



* Xouvclle Methode, &c, I, Introd. p. VJ. I am indebted to Mr. Cresson for 

 calling my attention to this passage in Jurine. The genera especially referred 

 t'> by this writer, us having an obvious system of bullae, are Nomada and Andre- 

 na. The bullae are tolerably plain also in Cerceris, Philanthus, Astata, Sphex, 

 Priononyx, Zethus, Augochlora, Epeolus and Macrocera, and in many other Aeu- 

 1 iate genera there are more or less plain vestiges of them. It is singular that 

 Jurine in his text states that the number of bullre in Hymenoptera varies from 

 one to seven, (exclusive of course of F and G which he had entirely overlooked.) 

 while iii the figure which he gives he correctly represents the bullae on the sub- 

 marginal cross-veins and recurrent veins of Andrena and Nomada as eight in 

 number. (Plate V. case 15.) He is incorrect in asserting that the continuity 

 of the exterior t u 1 f the vein is interrupted at the point where the bulla oc- 

 curs. The transverse striations on the exterior of the vein may be distinctly 

 traced under a high power throughout the bulla. «-»Ti 



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