62 THE SCOTTISH NATURALIST 



system of muscles at the back of the clypeus, radiating from 

 a plate placed under the anterior part of the oesophagus.^ 

 The egg-breaker forms no part of the embryo, for it belongs 

 to the amnion skin, and is shed with it immediately after 

 hatching, and the amnion is to be regarded as the innermost 

 layer of the egg-shell, rather than the outermost skin of the 

 embryo. 



For some days before the hatching of the eggs, occasional 

 movements of the mouth-parts may be observed through the 

 semitransparent labrum, also — especially when the eggs 

 were suddenly subjected to brilliant transmitted light under 

 a microscope — contractile movements at the head-end. 

 When hatching is about to commence a rhythmical move- 

 ment of the mouth-parts begins, and immediately afterwards 

 is set up a pulsating, alternately expansive and contractile 

 movement of the tissue within the epicranial notch, where 

 the upper end of the file is attached. This causes the file to 

 rub ao-ainst the egg-shell, which in a few minutes ruptures. 

 The head of the larval Psocid protrudes and the tail-end 

 comes to touch the base of the egg-shell. The body next 

 bends over the front of the egg, and the observer gets a view 

 for the first time of the eyes and epicranium. The latter is 

 much more arched than it becomes after complete hatching, 

 and viewed from the anterior apex the body is at this time 

 fairly cylindrical. By continued wormlike movements the 

 whole larva gradually emerges and then lies still, like a pupa, 

 in the amnion skin (Fig. 4), except for the pulsations at the 

 head which still move the file, while the rhythmical mouth 

 movements become increasingly vigorous. Large bubbles of 

 air are now seen moving from the mouth inwards, inflating 

 the alimentary canal as far as the third abdominal segment. 

 The antennae and legs, although in sheaths of their own, are 

 useless and inextensible. In about fifteen minutes the 

 shedding of the amnion skin commences by a split between 



1 Burgess {Pr. Boston Sac, xix., 178), who studied the mouth-parts 

 of Psocids, calls this plate "the oesophageal bone," and the muscles the 

 " clypeal muscles." I do not think the latter assist in the movement of 

 the hatching spine, and as they persist throughout adult life this seems 

 the more unlikely. 



