246 



PSYCHE. 



[September, 1901 



STUDIES FOR STUDENTS. — II. 



THE IIISTOBLASTS (iMAGINAL BUDs) OK THE WINGS AND LEGS OF THE GIANT CRANE- 

 FLY i^Holoruiia riibigitiosa). 



BY VERNON L. KELLOGG, STANFORD UNIVERSITY, CALIF. 



Tccliiiical note. — In an old larva of 

 Holoi-iisia (or other Tipiilid) killed as 

 described in Studies for Students, — I, 

 (Psyche, 1901, p. 207,) make a longitu- 

 dinal cut through the dorsal body wall 

 from head to posterior tip of body and 

 pin out in dissecting dish with ventral 

 aspect down. Remove the fat body and 

 alimentary canal with salivary glands, 

 exposing completely the longitudinal 

 and annular muscle fibers of the body 

 wall. In the thoracic segments carefully 

 pick away these muscle fibers, thus ex- 

 posing six small white bodies lying, one 

 pair in each thoracic segment, in imme- 

 diate contact with the skin. These are 

 the histoblasts (imaginal buds) of the 

 wings and legs of the imago. 



Position, and External Characters. 



The histoblasts, imaginal buds or 

 imaginal discs, as they are variously 

 termed, of the wings and legs of the 

 imago of Holorusia are readily found in 

 the old larvae. They are small, firm, 

 white bud-like bodies* (fig. i, A) lying 



* For the reason that some of these " imaginal " buds 

 are in many insects the beginnings of strictly pupal 

 organs which are not present in the imago, I believe that 

 the name imaginal buds or discs should be discarded. 

 The name, histoblasts, used first, I believe, by Kiinckel 

 d'Herculais, seems preferable. 



in the thoracic segments, against the 

 hypoderm (cellular skin layer of the body 

 wall) of which they are actual invagi- 

 nations. There are two pairs in each 

 segment, those of the prothorax being 

 the beginnings of the fore legs and of 

 the pupal dorsal prothoracic respiratory 

 tubes, those of the mesothorax corre- 

 sponding to the mesothoracic legs and 

 wings, and those of the metathorax to 

 the metathoracic legs and halteres. In 

 the dissection as made, the inner or 

 mesal buds are those of the legs, the 

 outer or lateral ones being those of the 

 respiratory tubes, wings and halteres. 

 The buds of the respiratory tubes and of 

 the balancers are smaller than the 

 others. 



By examination under the microscope 

 it may be seen that each bud is con- 

 nected intimately with the hypoderm by 

 a very short iicck. The buds may be 

 swayed slightly to one side or the other. 

 If care is taken in dissecting away the 

 viscera and muscles to expose the buds, 

 each will be noted to have a trachea! 

 tube running to it. 



Histology and Development. 



For a study of the structure and of 

 the development of the histoblasts it is 

 necessary to make sections (by micro- 



