240 BOWERBANK ON 



in considerable numbers in small pools of water in boggy or 

 marshy places ; mine were procured from similar situations on 

 Hampstead Heath. They may be preserved for many montlis 

 in glass jars, or other vessels, with a little duck-weed floating 

 upon the surface of the water. In selecting them for the 

 purpose of exhibiting the circulation of the blood, care should 

 be taken to choose such as have not yet attained a greater 

 length of body than about one-eighth of an inch, as the whole 

 insect may then be subjected to examination, with a reason- 

 able expectation of seeing the full extent of the great dorsal 

 vessel ; particularly if one be selected which has the intestinal 

 canal free, or nearly free, from food, as the success of the 

 observation will greatly depend upon this circumstance. In 

 fixing the larva for observation, which of course must be in 

 water, great care must be taken not to compress the body, as, 

 although the central circulation may be seen proceeding with 

 considerable vigour, that through the lateral vessels, and those 

 of the tail, legs, and antennae, will either be much impeded or 

 entirely stopped. Having fixed the insect, with the above 

 precautions, and with its back towards you, a truly beautiful 

 and astonishing sight presents itself. The blood, abounding 

 in flattened oat-shaped particles, will be seen circulating in 

 every part of the body, not in a continuous stream, but at 

 regular periods, agreeing in its motion with the pulsations of 

 the great dorsal vessel. This vessel (Fig. 1, a, a, a, a,) ex- 

 tends nearly the whole length of the body, and is of great 

 comparative magnitude. It is furnished, at regular intervals, 

 with double valves, about equal in number to the sections of 

 the body. Both above and below each of these sets of valves 

 is a pair of singular looking appendages, (Fig. 2, a.) They are, 

 probably, nervous ganglions, auxiliary to the motions of the 

 vessel, but so exceedingly pellucid as scarcely to be defined 

 with the highest power which can be applied to them. The 

 action of the valves is a most interesting and beautiful sight. 

 While in their greatest state of collapse, the point of the 

 lower valve is seen closely compressed .within the upper one, 

 (Fig. 3, a.) At the commencement of the expansion of the 

 artery, the blood is seen flowing in from the lateral apertures, 

 (Fig. 2, b) ; and, at the same time, the stream in the artery com- 

 mences its ascent. When it has nearly attained its greatest 

 state of expansion, the sides of the lower valve are forced 



