92 Mt. a. S. Packard on the Development 



an attempt has been made to trace these parts in the rings of 

 the head by those who have proposed theories of the number 

 of arthromeres in the head of insects. 



As we can understand the structure of the thorax better after 

 studying the abdomen, so we can only homologize the different 

 •head-pieces after a careful study of the thorax of Insects and 

 the cephalothorax of Crustacea, which thus afford us a standard 

 of comparison. 



Since the arthropleural is the limb-bearing region in the 

 thorax, it must follow that this region is largely developed in 

 the head, to the bulk of which the sensory and appended diges- 

 tive organs bear so large a proportion ; and as all the parts of 

 the head are subordinated in their .development to that of the 

 appendages of which they form the support, it must follow logi- 

 cally that the larger portion of the body of the head is pleural, 

 and that the tergal and, especially, the sternal parts are either 

 "very slightly developed or wholly obsolescent. Such we find to 

 be the fact. As to the number of rings composing the head, it 

 is evident that it is correlated with the number of appendages 

 they are to support. Hence, as in the thorax there are three 

 rings bearing three pairs of appendages or legs, it follows that 

 in the head, where there are seven pairs of appendages, there 

 must be seven rings. That there are seven such appendages, 

 among which we would include the eyes, which, if not homolo- 

 gous with the limbs, or, more properly speaking, repetitions of 

 the ideal appendage, are at least their equivalents, in that they 

 are situated on a distinct ring, as are the ocelli, which are exact 

 equivalents or repetitions of the eye, is evident. 



The larvse of Ephemera and Libellula, in the head of which 

 these parts of the cephalic rings, by reason of the degradational 

 character of the insects, appear in their simplest forms, afford 

 us the best material for study. In the head of the larva of 

 Libellula we have observed that the greatly elongated labium, 

 masking, when at rest, the mandibles, is in reality composed of 

 three sternites, immersed in and surrounded by three pleurites, 

 all bearing appendages, the basal pair being the mandibles, the 

 middle pair maxillse, and thirdly, the pair of labial palpi, all of 

 which are placed behind the mouth-opening. Beyond and in 

 front of the mouth are successively placed the sensory organs, 

 the antennae, the pair of eyes, and what we must consider 

 two pairs of ocelli, since the early forms of Ephemera and the 

 early stages of Bombus show the three ocelli resting on three 

 separate pieces, the two posterior pieces {pleurites) forming a 

 pair, while the single ocellus in advance is placed on a trian- 

 gular piece which we regard as two pleurites united on the 

 median line of the body, as the ocellus has a double form, being 



