MICROSCOPIC OBJECTS. 61 



Fig. 121. Water-weed Beetle (Helophorus gramilaris), x 12. 



This little beetle may often be noticed darting about 

 amongst the weeds in ponds, and may also often be found in 

 the standing water of old water-butts, more especially if the 

 butt has been left in an exposed and sunny spot. The follow- 

 ing is a description of the order Coleoptera : ' ' Wings four ; 

 anterior (elytra) hard, coriaceous, or horny, covering the 

 posterior, which are membranous and transversely folded ; 

 mouth formed for manducation, furnished with mandibles, 

 maxillae, and palpi : metamorphosis complete." 



This beetle may be mounted in a transparent manner. 

 Soak the insect in a moderately strong solution of potassa 

 fusa until it is transparent, then well wash in warm water, 

 and dry under pressure between glass slips ; soak in pure tur- 

 pentine, and mount in balsam as usual. 



Fig. 122. Spiracle of Great Water-Beetle (Dytiscus mar- 

 ginalis) , x 40. 



Spiracles are the breathing-pores of insects, of which this 

 drawing is an example. They supply them with the air 

 necessary for the purification of the blood, which in the case 

 of most animals passes through the mouth, but in insects it 

 is carried through tubes called tracheae, which are joined to 

 the spiracles. The spiracles of the various orders of insects 

 differ but slightly in their general form, being principally oval 

 or circular ; at their orifices hairs are generally placed to pre- 

 vent foreign substances, such as dust &c., from entering into 

 the minute ramifications of the tracheae. Spiracles are mostly 

 found on the abdomen, and also on the thorax of insects ; in 

 the Greater Water-Beetle they are found under the wings. 



The mode of preparation is to soak the skin containing the 

 spiracle for a short time in diluted potassa fusa, wash in warm 

 water, dry under pressure, soak in turpentine, and mount in 

 Canada balsam. 



