SEGMENTATION. 



609 



Heymons. * 

 "Acron." 

 1 Segment (Pre-antennal) bears the eyesf 



2 

 3 

 4 

 5 

 6 

 7 

 8 

 9 



10 

 11 

 12 

 13 

 14 

 15 

 16 

 17 

 18 

 19 

 20 

 21 



bears Antennae 

 disappears in embryo 

 bears Mandibles 

 1st Maxillae 

 2nd Maxillae 

 1st Thoracic legs \ g 

 2nd Thoracic legs ' 

 3rd Thoracic 

 = 1st Abdominal 

 = 2nd Abdominal 

 = 3rd Abdominal 

 = 4th Abdominal 

 = 5th Abdominal 

 = 6th Abdominal 

 = 7th Abdominal 

 = 8th Abdominal 

 = 9th Abdominal 

 = 10th Abdominal 

 = llth Abdominal 

 = " Anal piece " 

 "Telson" 



Lankester and Folsom. 



Eyes f (pre-antennal) 



Antennae 



Disappears in embryo 



Mandibles 



Superlingual Maxillulae 



1st Maxillae 



2nd Maxillae 



1st Thoracic legs \ g 



2nd Thoracic legs ! o 



3rd Thoracic legsj {; 



1st Abdominal ^ 



2nd Abdominal 



3rd Abdominal 



4th Abdominal 



5th Abdominal 



6th Abdominal 



7th Abdominal 



8th Abdominal 



9th Abdominal 



10th Abdominal 



1 1th Abdominal 



Heymons does not recognize the superlingual segment bearing 

 the maxillulae, described by Folsom in the Apterygota. . This 

 segment is said to supply the dorsal element in the hypopharynx, 

 a median structure or " tongue " which lies between the maxillae. 

 He places the division between the head and thorax at the junc- 

 tion of , the 6th and 7th segments, a position which corresponds 

 with the division between the tagmata in the Crustacea. The 

 Odonata, certain Orthoptera and the Dermaptera (earwigs), 

 and Lepidopterous embryos show eleven abdominal segments, 

 and, except in the Collembola, where the number is six, In- 

 sects have not less than ten abdominal segments. 



It is interesting to note that both views assign twenty-one 

 segments as the full complement for an Insect since this number 

 has been asserted for the higher Crustacea and Arachnida. } In no 

 Insect however can twenty-one segments be seen at one time. 

 The appendages of the third disappear (Campodea is a probable 



* Zoologica, 1901, p. 142. Heymons recognizes median anterior and 

 posterior portions which he calls the " acron " and " telson " respectively ; 

 he calls the acron plus the pre-antennal segment the protocephalon. 



f See also table on p. 325. 



J In the lower Crustacea, and of course in the Myriapods the number of 

 segments is greater, so that the significance of the identity of number 

 in the higher forms is not apparent. 



Z HI R R 



