The Great Water-Beetle. 425 



the head to the bottom of the trough by means of a strong 

 pin. Now we will fill the trough with water, and our subject 

 is ready for dissection. Let us first of all remove the elytra, 

 taking notice of the circular membranes at their bases, as before 

 alluded to ; we then unfold the wings, and with the scissors 

 nip them off at their juncture with the thorax, disclosing the 

 abdomen with its beautiful covering of glossy brown hair. 

 We can now count the spiracles on each margin of the body, 

 which we may observe are placed rather high, so as the more 

 readily to admit the air that fills the cavity formed by the 

 enveloping elytra or wing cases. A magnified figure of a 

 spiracle is shown in the engraving. We will next insert the 

 point of the scissors at the tail extremity, and cut upwards, in 

 a line with the axis of the body, as far as the thorax, then make 

 a tranverse cut on each side, and turn the membrane back, 

 holding it with the forceps. What a magnificent spectacle to 

 be sure ! Look at those threads which run up the inner 

 surface of the membrane, bright as silver. See how innu- 

 merable are these delicate fibres which run in all directions. 

 These we recognize as the tracliece, or respiratory apparatus 

 of the insect ; and we cannot take a single portion of the 

 beetle and submit it to a microscopic examination without 

 seeing some of these beautiful silvery fibres. Let us cutaway 

 these abdominal segments so as to expose the internal parts 

 to view. We observe between the skin and intestine a thick 

 network of fatty matter investing every organ, bound together 

 by the delicate tracheas. We will remove as much as we can 

 of this adipose matter, and try to discover the intestinal canal. 

 What is this body covered with numerous little villosities ? 

 It is the stomach, which we will seize gently with the forceps, 

 and then trace downwards to the anus, taking care not to 

 break the delicate structure. Having freed this portion of 

 the tube, we will proceed upwards to the head and mouth ; 

 cutting through the thorax and using much care lest we sever 

 the membrane. 



We have succeeded, let us suppose, in dissecting out the 

 entire intestinal canal. We must now float it out in another 

 trough, or suspend it by a thread in a test-tube and examine 

 it. First, then, we notice at its upper extremity an expanded 

 portion — this is the 'pharynx, the distended commencement of 

 the oesophagus, which latter portion of the tube reaches to a 

 small bulbous-like organ, in form resembling the cup of an 

 acorn (see Fig. 1). The part of the tube just above this 

 bulbous body is seen to expand into a sack-shaped crop 

 (ingluvies). In this crop the food is first prepared. If a 

 Dyticus be opened soon after a meal, the ingluvies will be seen 

 to be distended with the food. From the crop the nutritive 



