DEVELOPMENT OF INSECTS lOO? 



eggs from which eggs none but drones are ever produced. From careful 

 microscopic examination of the drone eggs laid even by impregnated 

 queens, Siebold drew the conclusion that they have not received the 

 fertilising influence of the male fluid, which is communicated to the 

 queen-eggs and worker-eggs alone ; so that the products of sexual 

 generation are always female, the males being developed from these 

 by a process which is essentially one of gemmation. 1 



The embryonic development of insects is a study of peculiar 

 interest from the fact that it may be considered as divided (at 

 least in such as undergo a ' complete metamorphosis ') into two 

 stages that are separated by the whole active life of the larva that, 

 namely, by which the larva is produced within the egg, and that by 

 which the imago or perfect insect is produced within the body of 

 the pupa. Various circumstances combine, however, to render the 

 study a very difficult one ; so that it is not one to be taken up by 

 the inexperienced microscopist. The following summary of the 

 history of the process in the common blow- fly, however, will pro- 

 bably be acceptable. A yaetmila with two membranous lamellae 

 having been evolved in the first instance, the outer lamella very 

 rapidly shapes itself into the form of the larva, and shows a well- 

 marked segmented division. The alimentary canal, in like manner, 

 shapes itself from the inner lamella, at first being straight and 

 very capacious, including the whole yolk, but gradually becoming 

 narrow and tortuous as additional layers of cells are developed 

 between the two primitive lamellae, from which the other internal 

 organs are evolved. When the larva comes forth from the egg it 

 still contains the remains of the yolk ; it soon begins, however, to 

 feed voraciously ; and in no long period it grows to many thousand 

 times its original weight, without making any essential progress in 

 development, but simply accumulating material for future use. An 

 adequate store of nutriment (analogous to the * supplemental yolk ' 

 of Purpura) having thus been laid up w r ithin the body of the 

 larva, it resumes (so to speak) its embryonic development, its passage 

 into the pupa state, from which the imago is to come forth, involving 

 a degeneration of all the larval tissues ; whilst the tissues and 

 organs of the imago 'are redeveloped from cells which originate 

 from the disintegrated parts of the larva, under conditions similar 

 to those appertaining to the formation of the embryonic tissues from 

 the yolk.' The development of the segments of the head and body 

 in insects generally proceeds from the corresponding larval segments ; 

 but, according to Dr. "Weismann, there is a marked exception in the 

 case of the Diptera and other insects whose larvae are unfurnished 

 with legs, their head and thorax being newly formed from ' imaginal 

 discs,' w^hich adhere to the nerves and tracheae of the anterior 

 extremity of the larva ; 2 and, strange as this assertion may seem, 



1 See Professor Siebold's memoir, On True Parthenogenesis in Moths and Bees, 

 translated by W. S. Dallas (London, 1857) ; and his Beitrage zur Parthenogenesis 

 der Arthropoden (Leipzig, 1871). 



- See his ' Entwickelung der Dipteren ' in Zeitsclirift f. Wiss. Zool. xiii. and xiv. ; 

 Mr. Lowne's Anatomy of the Blow-fly (1st ed.), pp. 6-9, 118-121 ; and A. Kowalevsky, 

 ' Beitrage zur Kenntnis der Nachembryonalen-Entwickelung der Musciden,' Zeitschr. 

 f. Wiss. Zool. xliv. p. 542. 



