AMPHIBIOUS INVERTEBRATES 463 



AMPHIBIOUS NET-WINGED INSECTS (Neuroptera) 



The most remarkable structures by which amphibious insects 

 are enabled to breathe the air dissolved in water are those known 

 as tracheal gills, which may be briefly defined as gills traversed 

 by air-tubes (trachece) that do not open to the exterior. Gills of 

 this sort are found in the early stages of many net-winged insects, 

 but are rare in the adults. They are clearly of secondary nature, 

 i.e. do not correspond to the gills of an ancient ancestral stock, for 

 the air-tubes which they contain are structures evolved with refer- 

 ence to air-breathing (see p. 434), and are here pressed into the 

 service, as it were, of breathing in water. The gill itself is an 

 outgrowth of the body which offers a large surface for exchange 

 of gases between the air in the air-tubes and that dissolved in 

 the surrounding water. The evolution of an aquatic mode of life 

 requires a very long time, especially when it takes place in so 

 typical a class of land animals as insects, and even without definite 

 evidence it would be reasonable to suppose that the amphibious- 

 ness of certain insects has taken longer to come about than the 

 acquirement of an aquatic habit by the insects, elsewhere described, 

 which are not able to breathe air dissolved in water. There is, 

 however, positive geological evidence to show that the tracheal 

 gills of net-winged insects are structures of very great antiquity. 

 Numerous fossils belonging to this group have been found in 

 rocks belonging to that immensely remote period to which the 

 name Carboniferous has been applied because its luxuriant vege- 

 tation has been converted into those coal-deposits which are of 

 greatest importance. Many of the extinct insects of that period 

 belong to existing groups of Net -Wings, while others, though 

 referable to this order, constitute groups which have no living 

 representatives. One of these insects {Corydaloides Scudderi) 

 is especially interesting, for, when adult, it possessed tracheal 

 gills resembling those found in the early stages of May- Flies, 

 such as will be described later on. 



We will now consider the amphibious members of certain 

 families of Net-winged Insects. These are: Stone- Flies (Per- 

 lidse), Dragon-Flies (Odonata), May-Flies (Ephemeridee), Alder- 

 Flies (Sialidse), and Caddis-Flies (Phryganeidae). 



Stone -Flies {^Perlidce). — These insects, of which about two 

 dozen British species are known, live in rapidly-flowing streams 



