1864.] 543 



On the Insects. COLEOPTEROUS, HYMENOPTEROUS and DIPTEROUS, in- 

 habiting the Galls of certain species of Willow. 



Part 1st.— DIPTERA. 

 BY BENJ. D. WALSH, M A. 



I propose in the following pages to name and describe the Galls, 

 which I have found on several species of Willow in the neighborhood 

 of Rock Island, Illinois, and also the insects which produce those 

 galls, not only in the imago state, but in all their states so far as known 

 to me. I propose at the same time to name, and, so far as they are 

 hitherto undescribed, to describe several other insects, which habitu- 

 ally breed in the galls formed by the true gall-makers, and which, as 

 they feed on the substance of the gall itself and only occasionally or inci- 

 dentally destroy the gall-making insect, may be appropriately considered 

 as Inquilines or Guest-flies. Besides these last, there is a great variety 

 of true Parasites, mostly Chalcididae, which prey, not on the gall, but 

 solely and exclusively on the body of the Gall-maker or on that of some 

 of the Inquilines, and which I shall only refer to so far as they are 

 concerned with the other subjects herein discussed. The field thus 

 opened to our view, though very extensive, is almost an untrodden one; 

 for out of the great multitude of N. A. willow-galls, but two, so far as I am 

 aware, have been up to this day named and described, viz. SaUcis Fitch 

 { = rigiJse 0. S.) and strohiloides 0. S.; and in the case of the latter, the 

 insect that produces it has hitherto been totally unknown in all its states. 



As in my other published descriptions, I have wherever possible de- 

 scribed from a large number of specimens and carefully given all the 

 variations, so as to define the species itself and not merely the indi- 

 vidual, stating in every case the number of specimens as a measure of 

 the value of the description. I have also, as heretofore, aimed at mak- 

 ing the descriptions as accurate and definite as possible, and with this 

 object in view have uniformly sacrificed brevity to precision. To the 

 moi'e advanced student, perhaps, this is not always desirable ; but to 

 the neophyte what information does it convey to say, for example, 

 " Antennal joints spherical, pedicels short, verticils long," when he 

 knows not how long the pedicels and verticils usually are ? Whereas 

 if we say, " Antennal joints spherical, pedicels I as long as the spherical 

 part of each joint, verticils as long as the two entire joints from which 



