358 



IXSECTA. 



2. Hemimetabola. The young stage differs from the imago, not 

 only in the absence of wings, but also in the presence of provisional 

 (larval) organs. 



The larvae live in water and differ from the imago in the condition 

 of their respiratory organs, the former possessing tracheal gills, either 

 external or internal (intestinal respiration) in position. 



To this group belong the Ephemeri- 

 dae, the Odonata, and the Plecoptera. 



The Ephemeridae represent a very primitive 

 group. In them alone have the paired efferent 

 ducts of the genital organs heen retained in 

 their original form. The larvae (Fig. 177) 

 live in water, ami leave the egg in a Campodei- 

 form stage.* In the later stages, they are 

 distinguished by the possession of external 

 tracheal gills (k), which may be leaf-shaped 

 or tufted, etc., and are usually attached 

 to the postero-external margin of the terga 

 of the seven anterior abdominal segments. 

 Within these integumental outgrowths, 

 richly-branched tracheal trunks extend, and 

 here the exchange of gases with the sur- 

 rounding medium takes place. In keeping 

 with the aquatic manner of life, the stig- 

 mata are closed, and the stigmatic tracheae 

 found in the meso- and meta- thorax and 

 in the eight anterior abdominal segments 

 are merely thin strands cpiite devoid of air 

 (dosed tracheal system). Only at the moment 

 of hatching do the stigmatic tracheae and the 

 stigmata open, so as to allow of the passage of 

 the tracheal intima which is shed with the 

 body-cuticle (Palmen, No. 161). The num- 

 ber of moults which mark the successive 

 stages through which the larva gradually 

 approaches the imaginal form is very con- 

 siderable (in Chloe over twenty, Lubbock). 

 The last moult but one {i.e., the last larval or nymph-stage) in which imperfect 



* [There are two types of Insect larvae: the Campodeiform, so-called from its 

 resemblance to the Thysanuran Campodea, and the eruciform. In the first we 

 find the three typical regions of the body clearly defined, biting mouth-parts, 

 ambulatory thoracic limbs, and sometimes terminal abdominal appendages. 

 This larva is characteristic of the Ametabola and the Hemimetabola. In the 

 second type the head is usually well-defined, but the body-segments are simple 

 and cylindrical, and the animal has a vermiform aspect, the mouth-parts are 

 usually adapted for biting, but may be much reduced, the thoracic limbs are 

 usually present together with functional abdominal appendages, the "prolegs." 

 The cruciform larva is very generally found among the Heteromorpha ; it attains 

 its most characteristic development in the Lepidoptera, while in the Diptera 

 it is much modified and degenerate, being apodal. — Ed.] 



Fig. 177 — Ephemeriil larva, k, tra- 

 cheal gills ; t, principal trunks of 

 the tracheal system. 



