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



appendages attached to the head in any case (in Crustacea), six 

 segments composing the head of Arthropoda, — the first bear- 

 ing the eyes, the second and third the two pair of antennas, and 

 the fourth, fifth, and sixth the three pairs of maxilla? (the third 

 of which is soldered to the lower lip in insects). For insects, he 

 reduces the number to five, as they have never more than one 

 pair of antennas. 



In seeking for some arguments against this conclusion, I may 

 be allowed to start, not from the head provided with the greatest 

 number of appendages, but from the head of insects, — first, be- 

 cause the statement that a pair of appendages is the exponent 

 of a separate segment may be considered as a petitio principii 

 by one who desires to disprove it; and secondly, because it is 

 only in insects that the head is a section of the body into which 

 nothing but the head itself enters. In Crustacea, either a part 

 of the thorax (as in Isopoda) or the whole thorax (as in Deca- 

 poda) is intimately united with the head into one portion ; and 

 in Arachnida there is no proper head at all. 



In the head of insects we have five pairs of appendages, if we 

 admit, from the analogy of the moveable eyes in the Podoph- 

 thalmous Crustacea, that the sessile eyes are appendages com- 

 parable to the antennas and maxillas. As it is proved, by the 

 second and third thoracic segments bearing wings and legs at 

 the same time, that the same segment may be provided with a 

 pair of both tergal and ventral appendages, we might at first 

 conclude that the eyes and antennas are the tergal appendages 

 of the same segments which bear maxillae as their sternal ones. 

 The number of the segments of the head would thus be reduced 

 to three, which is, indeed, a conclusion admitted by some ento- 

 mologists*. But Prof. Huxley has been led, by his observations 

 on the development of Aphis, to the conclusion that both eyes 

 and antennas are also sternal, and not tergal appendages. 

 With regard to these observations, I cannot refrain from men- 

 tioning that scarcely any object could be chosen which offers a 

 greater difficulty to the observer for seizing clearly this fact 

 than Aphis ; for the Aphides not only leave the egg in a com- 

 paratively perfect state, undergoing afterwards scarcely any 

 metamorphosis, but they have also the front and even the vertex 

 bent downwards, so as to be visible on the underside of the head. 

 How is it to be decided, in such an embryo, which is the sternal 

 and which the tergal part of the two segments, which, according 



* The labrum is considered by some entomologists, as by Brulle (Ann. 

 des Sc. Nat. 1844, p. 345), as representing also a pair of soldered maxillae 

 lying above the mandibles : this analogy is, however, completely rejected 

 by Prof. Huxley, on the ground that the labrum is developed in the medial 

 line of the body (I. c. p. 232. 9). 



