CHAPTER XIV 



EPHEMERIDA, ODONATA, PLECOPTERA, EMBIARIA, 

 DERMAPTERA, HEMIMERINA 



EPHEMERIDA 

 The May Fly (Ephemera vulgata L.) 



The eggs of Ephemera vulgata are ovoid, about 0.3 mm. in diameter, 

 and covered with a gelatinous substance which swells when deposited in 

 the water. The developmental period, at a temperature of 20 to 25°C., is 

 about 10 or 11 daj^s. The germ band arises at the posterior end of the 

 egg. As development progresses, the posterior end of the germ band is 

 pushed into the yolk until it reaches the anterior end of the egg and then 

 with a further increase in length becomes S-shaped in the yolk. The 

 embryo is thus of the immersed type, resembling that of the damsel flies. 

 The movement in blastokinesis likewise takes place as with the damsel fl,y, 

 whereby the embryo assumes a position on the ventral surface of the egg. 



Segmentation of the embryo, according to Heymons (1896c), occurs 

 while it is still immersed in the yolk. On the abdomen appear 1 1 pairs of 

 small, flat appendages the last of which will develop into the pair of caudal 

 filaments (cerci). The median filament is the elongate tergite of the 

 eleventh abdominal segment. 



The tracheal system, which arises as in terrestrial insects from lateral 

 ectodermal invaginations, does not contain air at the time of hatching and 

 is not fully developed until later. The mid-gut in the newly hatched 

 nymph is still filled with yolk; the stomodaeum and proctodaeum, on the 

 other hand, are well developed, the latter with two Malpighian tubule 

 evaginations. Gonads are feebly differentiated during embryonic life. 



According to Heymons (1896a), the serosa in the Ephemeridae consists 

 of large pavement cells; the amnion, however, is made up of small cells 

 similar to those of the germ-band from which it is clearly derived. 



Murphy (1922) makes the observation that in the embryo of Baetis 

 posticatus the early appearance of the inner lobe of the mandible and the 

 fact that it is articulated but has no musculature suggest the possibility of 

 a primitive structure. It occupies the same relative position on the 

 mandible as the lacinia mobilis of certain Crustacea {My sis, Arolana). 

 Embryological evidence indicates that the movable inner lobe of the 

 May-fly mandible is a lacinia. A mandibular palp is present at no stage 

 of embryonic development. Ephemera vulgata used by Heymons for his 



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