28 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8o 



EMBRYONIC REDUCTION OF ABDOMINAL SEGMENTS 



Of much interest and of somewhat uncertain significance is the 

 embryonic reduction of abdominal segments. It has appeared to be 

 a general law that in insects the maximum number of abdominal seg- 

 ments is to be found at the time of hatching from the egg ; and that 

 postembryonic changes, if there are any, are toward the reduction 

 in the number possessed at hatching. 



The Protura. of all the hexapods, show both a reduced number of 

 abdominal segments (nine) at hatching and add segments during 

 postembryonic development. It was largely for this reason that Ber- 

 lese ( 1910) regarded them as occupying a position between the true 

 insects and the myriapods and gave to them the name Myrientomata. 



The Collembola show a reduced number of abdominal segments 

 at hatching (six) but do not add segments during their postembryonic 

 development. 



It may be that in both of these groups, the Protura and the Collem- 

 bola, the reduction during the embryonic development of the number 

 of abdominal segments is an inherited condition from myriapodlike 

 ancestors and, if so, should indicate a closer affinity between these 

 groups than has been suspected. In this connection, however, it should 

 be noted that some or all of the abdominal segments of Collembola 

 may in certain genera remain fused throughout life. This suggests 

 the possibility of an adaptive reduction during the embryonic life, 

 possibly during a deutovum stage. Such stages, or instars, inside of 

 the eggshell are very generally met with in arthropods and are typical 

 of arachnids. A reduction of abdominal segments during a deutovum 

 or a tritovum stage or during both of these stages should not be 

 regarded as in any way indicating a primitive condition for insects. 

 The deutovum and tritovum instars are comparable to the proto- 

 n}'mphs and deutonymphs of arthropods undergoing a normal de- 

 velopment. 



THE TERGAL PLATES OF THE LEG-BEARING SEGMENTS 



Examinations of insect larvae and apterygotan insects have re- 

 vealed the presence of a very simple dorsal plate, or tergum, for most 

 of the body segments. The simplest type consists of an undivided 

 plate, without ridges, sutures or apodemes, which does not fully 

 occupy the dorsal surface of the segment. Snodgrass has very fully 

 described the types of terga found in winged insects (Snodgrass, 

 1925). More recently he has described a type among wingless insects 

 (Snodgrass, 1927) which is considered as showing certain features 

 found in the thoracic terga of most winged insects. 



