THE ENTOMOLOGIST. 39 



into all the soft parts of the new organ, whether muscle, 

 trachea or tendon. In the formation of pupal air-vesicles, 

 however, a trachea fulfils this function. The external ap- 

 pendages of the imago, such as the wings and legs, are formed, 

 tliough not of course fully developed, during the larva stage ; 

 and Dr. Weissmann refers the position of the antennae to a 

 muscular act on their part, for which he brings forward 

 strong reasons. From a pupa of this kind to a pro-imago, 

 as it has been called, of Ephemera or Chloeon, there is but a 

 step, even if so much. Tn fact the so-called pupa of Corethra 

 ought to be called a pro-imago rather than a pupa. Its 

 functions, with one single exception, are in full activity. 

 Though it does not feed, it swims and appears as active and 

 vivacious as the larva. It can see perfectly well, and on 

 the approach of danger darts rapidly to a place of greater 

 security. In Corethra the larval segments develop them- 

 selves directly into those of the imago, and the appendages 

 of the head into the corresponding organs of the perfect 

 insect. The thoracic appendages are formed during the last 

 stage of the larva, by outgrowths of the hypodermis round a 

 nerve or a trachea, from the cellular envelope of which the 

 cellular tissues in the interior of the organ are formed. The 

 larval muscles in the abdomen are received almost unaltered 

 into the imago. The muscles peculiar to the imago develop 

 themselves, in the last larval state, from indifferent cellidar 

 bands, which are present even in the egg. The genital 

 glands date from the embryo, and develop gradually ; all the 

 other systems of organs pass directly, with little or no 

 change, into the imago. In Musca the thorax and head rise 

 independently from the corresponding parts of the hypo- 

 dermis of the larva, and the abdomen only through direct 

 alteration of the eight last abdominal segments. The thorax 

 and head develop themselves from " imaginal disks" which 

 have their origin in the embryo. First, after the formation of 

 the pupal barrel-like envelope from the cliitin skin of the 

 larva, the imaginal disks develop themselves into the head 

 and thorax. There is a destruction of all the organic systems, 

 which are re-formed at the exjicnse of the fat body. The 

 genital glands are present in the embryo, and develop them- 

 selves gradually. Tiie pupa condition is protracted, and the 

 insect is inactive. 



