Thorax in ivmged Insects. 153 



I shall take another opportunity of discussing the external structure of 

 the Arachnida, which remains as yet quite unknown. I shall merely 

 now state that the Acaridce are those in which the segments of the body 

 are most condensed or confluent, as the Macrourous Crustacea are those 

 Annulosa which appear in the imago state to offer the greatest regular 

 developement. A Scolopendra offers a construction which goes appa- 

 rently beyond the regular type, and thus such Ametahola are in zoology 

 natural monsters.* The larvse of winged insects have in general thir- 

 teen segments, indeed I know at present of no one exception. A cater- 

 pillar, for instance, has a head, three segments for the thorax, and nine 

 for the abdomen. The first three thoracic segments carry feet ; the seg- 

 ment immediately following, or the fifth of the thirteen, (which, as I 

 conceive, may in general be accounted to belong to the abdomen of in- 

 sects,) rarely possesses locomotive appendages,f but the next segment to 

 this, that is the sixth segment (reckoning the head as one), is supplied 

 with them in certain larvae, such as those of some Tenthredinidce, which 

 have twenty-two feet. The last seven abdominal segments very often 

 one or other carry spurious feet; and on the other hand, the body may be 

 quite vermiform, ^: that is without any feet whatever, as we know from 

 looking at the larvse of certain Ilymenoptera, Coleoptera, and Diptera. 



behind the rest of his work. Scarcely a word, for instance, is said respecting 

 the class of Crustacea, and yet some account of their forms appears absolutely 

 necessary in an Introduction to Entomology. 



* Understanding well that every one of the thirteen primary segments of an 

 insect, when at the perfection of developement, is divisible, as will be shewn 

 in the following pages, into four minor segments, we get fifty-two segments 

 for an insect that is perfectly regularly developed, and this is the maximum 

 number in Chilognatha. The Chilopoda have only about half this number of 

 segments, owing to their primary segments in general being only about half as 

 much developed as those of the lulidcs. 



f In Crustacea, however, the fifth segment of the thirteen very commonly 

 carries feet, or locomotive appendages. 



\ In the " Hor<E Entomologicce" I followed the three greatest naturalists that 

 England has produced, Ray, Willughby, and Lister, in placing certain Vermes 

 among the Annulosa. A minute and careful examination of this subject has 

 convinced me of the accuracy of this mode of viewing nature. 



