176 Dr. H. Schaum on the Composition of the Head, 



to the segments. We find no trace of subsegments on the in- 

 tegument of the head in any stage of the development ; we find 

 no sets of difitrent muscles, and never more than one gavglion 

 infraoesophageum in the head. Are we not therefore entitled to 

 draw from these facts the conclusion that the head of insects 

 constitutes but a single segment, especially since we may trace 

 in many Coleoptera a dorsal and a ventral arcus of it, united in 

 two deep lines corresponding to sutures on the jugulum ? It is 

 certainly a segment more complicated than the others ; but 

 should we not expect this, since it contains the ganglion supra- 

 cesophageum, the principal organs of the senses, and the mouth*? 

 Applying the same views to the Crustacea, we are not entitled 

 to admit there, any more than in the Insecta, difi'erent segments 

 of the head for the difierent appendages, — not even in Squilla, 

 where the eyes and interior antennae are inserted, it is true, on 

 a separate plate, though a plate which is not analogous to a 

 true segment f. 



Keeping in view the requisites of a segment, we may also 

 arrive at a positive and satisfactory result as to the normal num- 

 ber of segments composing the abdomen of insects, which has 

 also been a subject of discussion in the memoir of Prof. Huxley 

 on the development of Aphis. This result is, that in no case 

 does the number of abdominal segments exceed nine. In insects 

 which undergo a complete metamorphosis, this is satisfactorily 

 proved by the fact that no larva has more than thirteen seg- 

 ments J, — the first being the head, the three following consti- 

 tuting the thorax, and the last nine the abdomen. Newport 

 (Todd's Cyclopsedia) and Westwood (Introd. to the Modern 

 Classif. of Insects, i. p. 194, and ii. p. 240) speak, indeed, of a 

 fourteenth segment of the body in the larvjE of aculeate Hy- 

 menoptera and of the Scarabseidse, in like manner as some 

 lepidopterologists speak of a fourteenth (anal) segment of cater- 

 pillars ; but it has long ago been proved by Erichson and Stein § 

 that this supposed tenth segment of the abdomen is nothing 

 but the externally protruded anus, analogous to the anal proleg 

 of the larvae of many Coleoptera (which no one ever thought of 

 considering a segment). As the number of segments does not 

 increase after the larva has escaped from the egg, we cannot 

 have more; than nine segments in any insect undergoing a 



* I may also mention that those who admit several segments have never 

 undertaken to state the position which the ganglion supraoesophageum oc- 

 cupies with regard to these segments. 



+ Erichson, Entomographien. Berlin, 1840, p. 17. 



X Some larvjE, as those of Dytiscida and Hydrophilidce, have, however, 

 but twelve (eight being abdominal). 



§ Verglcichende Anatomic der Insecten, i. p. 23, not. 4. 



