HARDWICKE'S SCIENCE-GOSSIP. 



169 



GRAPHIC MICROSCOPY. 



By E. T. D. 



No. VIII.— Spiracle of Breeze Fly (GEstrus equi). 



N elaborate and 

 exalted condition 

 of the organs of 

 respiration is, in 

 most cases, asso- 

 ciated with a high 

 development of 

 physical power 

 and strength. This 

 is notably seen in 

 insects ; their iin- 

 tiring flight, in- 

 cessant locomo- 

 tion, the muscular 

 force of their 

 various parts, 



nervous activity, 

 vivid life, sense 

 of smell, sight, 

 reproduction, and 

 generation of heat, 

 are functions evidently in close connection with a 

 capability of incessantly and perfectly incorporating 

 air with blood through an intricate mass of respi- 

 ratory vessels, more diffused than in any other living 

 thing. 



In insects there is no centralised breathing organ, 

 involving a system of veins and arteries ; instead of 

 blood seeking the lungs for aeration as in animals, 

 in these powerful creatures the air seeks the blood, 

 through tracheal tubes, inosculating and permeating 

 every part of the body, legs, wings, alimentary 

 canal and nerve centres ; and thus oxygen streams 

 through every vital part, admitted by external 

 apertures, "spiracles," plainly visible, on each side 

 of the abdominal segments of any insect, either in 

 the larval or perfect condition. These spiracles, or 

 "stigmata," are the entrances to this elaborate 

 respiratory system, freely admitting air. The 

 shape and character of the opening is diverse, 

 interesting to the microscopist, as, in distinct species 

 they are, in shape and character singularly dis- 

 No. 236.— August 1884. 



similar ; the typical condition is a contractible 

 puncture, with filamentous processes converging from 

 the margin to the centre, obviously to filter the 

 air, and prevent the admission of impurities. Some 

 are sieve-like, as in the common house-fly ; in larva 

 of many moths they take the form of a flexible slit or 

 compressible lip; in the blow-fly, an oval opening 

 with an elaborate screen : they are of various configu- 

 rations, circular, oblong, vermiform, crescent-shaped, 

 and all have sensitive edges expanding and con- 

 tracting under the influence of respiratory action. 



The spiracle of the breeze-fly of the horse {OEstriis 

 eqiii), the subject of the plate, and a popular micro- 

 scopical " slide," unlike what may be termed the 

 ordinary form of a single aperture, has a peculiarity 

 in a number of fine slits crossing a series of vermi- 

 form-like membranes, which, by their delicate elas- 

 ticity, open and close the fissures for the admission 

 and emission of air ; in the larva of the crane-fly 

 this condition is found arranged in a complete circle. 



Spiracles are the portals of the tracheae, a series of 

 delicate tubes ramifying through every part of the 

 body, and subdividing into microscopic filaments of 

 exquisite delicacy. In large insects, expanded at points 

 into dilatations or air-sacs, these vessels, imbedded 

 in the tissues, are preserved from collapse by a spiral 

 filament strong enough to resist compression and 

 yet sufficiently elastic to maintain the required ex- 

 pansion, analogous to the cartilaginous rings, 

 lining the tracheae leading to the lungs of animals. 

 This beautiful thread, enclosed between the 

 tissues of the tubes, may be traced in the minutest 

 terminations. The tracheae are supplied with the 

 ' ' breath of life " by, the respiratory palpitations of 

 the insect ; a rhythmical compression and distension 

 of the abdominal parts opens the spiracles for the 

 passing in and forcing out of air, quicker or slower, 

 synchronal with the pulsations of the body, increased 

 or retarded, under excitement or repose. The result 

 of this action may be seen by immersing a large 

 caterpillar, or beetle, in water ; air bubbles will then 

 emerge from the spiracles and cease when the 



