198 [September, 



EXPLANATION OF PLATES III-VI. 

 {HELOPHORINI). 



The plates consist in part of pliotographs and in part of drawings. We 

 began tliese illustrations with the intention of presenting ijhotograplnc repre- 

 sentations of the aedeagus in all the species, but grave difficulties were 

 encovxntered in securing satisfactory results, so that after some experience we 

 came to the conclusion that ovir purpose of giving an idea of the amount of 

 difference between the species of Ilelophorini was best obtained by means of 

 drawings made with the camera lucida. The student will be able to form an 

 opinion of the comparative merits of the two methods as we give a selection 

 from the photographs as well as the di'awings ; and we may call attention to the 

 fact that the drawing fig. 55 of plate V is made from the same preparation as the 

 photograph given on p. 236 of Vol. 51 (Heloph. affinis), but without any com- 

 parison at the time of making the drawing. The photographs were made by 

 Tams of Cambridge. The drawings are all dvie to my daughter, M. A. Sharp, 

 and have been made by means of a Bauscli and Lamb monocular microscope 

 and camera lucida. They are as nearly as may be on one scale of magnification, 

 which is approximately about ^. 



A general account has already been given (p. 27, Vol. 51) of the aedeagus, 

 and figs. 16 to 21 of pi. IV will serve to illustrate that, and to explain the 

 structures, in connection with the descriptions, given below, of the figures. But 

 it is necessary to remark that the great difficulty in these illustrations is tha.t 

 the membranes, which form a very important part of the meclianism, are per- 

 fectly transparent, and cannot be revealed by the figures, which in fact 

 represent the chitinous parts of a tiibular arrangement. The membranes that 

 svipplement the chitinoiis parts have to be supplied by the imagination of the 

 stitdent. 



A great difficulty exists in connection with the median lobe, as the apical 

 part (as will be seen by inspection of figs. 19 to 21) looks very different accord- 

 ing to tlie polut from which it is viewed, and also according to whether it is 

 completely closed or not. We have therefore not given so much attention to 

 this part as its real importance calls for. Note also the difficulties involved by 

 the difference between the dorsal and ventral rods, which is accentuated by the 

 fact that when the aedeagus is collapsed these rods lie one on the other, and 

 the eye cannot differentiate them. 



The special distinctions of the aedeagus of Helophorini are (1) the elongate 

 struts, and (2) the large size and pointed anterior extremity of the basal piece. 

 Helochares (a genus with extreme phallic peculiarities) has elongate struts, but 

 a small and differently shaped basal piece. Philijdrus is nearly without struts, 

 but has a somewhat similar basal piece. Chief importance may be attached to 

 the struts, as the basal piece is very subject to post-mortem shrivelling, and is 

 apparently somcAvhat variable in its scleritic extension. The encasement (i.e., 

 the highly modified last body segment) is extremely similar in all the forms 

 of Helophorini. 



