BACKBONELESS ANIMALS WHICH BREATHE IN AIR 439 



There is a very interesting kind of modification observable in 

 the breathing organs of Insects, such as Bees and Locusts, which 

 possess the power of rapid flight, for in such cases the air-tubes 

 swell into a varying number of air- 

 sacs (fig. 558), the result being to 

 give the body a larger bulk in pro- 

 portion to its weight than would 

 otherwise be the case. This is 

 curiously reminiscent of the state 

 of things in Birds, where the lungs 

 communicate with a system of large 

 air-sacs, and in both kinds of ani- 

 mal the use of the arrangement is 

 to be sought in connection with the 

 power of flight, which will be dis- 

 cussed in a later section. But it 

 must not be supposed that the air- 

 sacs are equivalent in the two cases, 

 for the lungs and air-sacs of Birds 

 arise by outgrowth from the diges- 

 tive tube, while the air-tubes and 

 air-sacs of Insects develop in the 

 first instance as simple in-pushlngs of the skin, which gradually 

 become more and more complicated until the adult condition 

 is reached. 



Air-breathing Aquatic Insects. — Some Insects live in 

 water during part or all of their existence, in spite of which 

 they may be dependent upon ordinary air for breathing pur- 

 poses. Special devices are then generally present, having re- 

 lation to the aquatic habit. The Great Water- Beetle [Dytiscus), 

 for example, lives entirely in the water during the larval part 

 of its life, and mostly so when adult. The elongated predaceous 

 larva has but one properly developed pair of breathing -pores, 

 placed at the tip of the tail, which is from time to time pro- 

 truded from the water to allow air to enter the respiratory tubes. 

 This is facilitated by the presence of numerous hairs at the hinder 

 end of the body, causing this region to float up more readily than 

 the head end. The adult Beetle is furnished with a full comple- 

 ment of breathing-pores, which open above into a space of which 

 the floor is formed by the upper side of the body, and the roof by 



Fig. 558. — Dissection of Honey-Bee {Apis mellifica), 

 enlarged 

 ss^ Two of the breathing-pores (stigmata), a.s., air- 

 sac, with which numerous branching air-tubes are 

 connected; f, feeler; f, eye; ///, legs; c, crop; st.^ 

 stomach ; ex. , excretory tubes ; r, rectum ; r.^., rectal 

 glands. 



