PROF. &GAS81Z8 



relative position within, and also the changes 

 which thev undergo during their growth, as well 

 as in their proportional development. To these 

 diagrams I shall mostly refer daring this illustra- 

 tion. But in such a comparison of structural dif- 

 ferences, the external arrangement of parts is as 

 mportant as I e internal differences, 



We have examined the forms of the various sta- 

 ges of growth in Insects. We have not examined 

 the differences in the arrangement of the external 

 parts. Let us begin the comparison with these. 



[See Plate IX, Lecture 6 J 



In the various Caterpillars or Maggots in the 

 various larvae of Insects which you see figured in 

 Plates IX, X and XI, and Plate II of the first lec- 

 ture, there is one form which is characteristic 

 in all which occurs universally in all. It is 

 the greater uniformity of rings when compared 

 to each other. The rings of the anterior part 

 of the body, (Plate X, fig. H, or Plate XI, fig. B) 



> r See Plate X. Lecture 6.] 



though here provided with legs, resemble the 

 rings in the middle portion of the body ; however, 

 they resemble these more closely than the anterior 

 rings resemble the posterior ones ; but as a whole, 

 considered in its general arrangement, the various 

 rings of the larvae are more uniform than in the 

 perfect insect, which arises from them ; and they 

 are naturally more uniform, but they are not 

 grouped together in any particular way. There 

 are no differences in the rings, indicating more 

 circumscribed parts of the body. Scarcely is the 

 head more defined from, the other rings by its co- 

 lor. But, between the so-called chest of Insects 

 and the abdominal region, there is no separation 

 (see Plates IX, X and XI) as we notice it in the 

 perfect insect. 



There is always in the perfect insect, between 

 the head and the chest, and the posterior part of 

 the body, a strong division, as we see in these fig- 

 ures. (Plate XI, figs. D and A) where the head is 

 more distinct; a certain number of rings consti- 

 tute another region behind the head, the so-called 

 thorax, or chest ; and behind this, there is a third 

 one the abdomen. Xow, such a division of rings 

 into distinct divisions into a head, thorax and 

 abdomen is not yet introduced into the condition 

 of the larva, though it is indicated by the appen- 

 dages; though not universally, but very general- 

 ly, there are among the anterior rings some which 

 have appendages more developed than the others, 

 which will correspond with the rings which form 

 the chest, and then the other rings behind will 

 correspond with the rings which form or constitute 

 the abdomen. 



But now, compare the proportional size between 

 those rings in a perfect insect Grasshopper for 

 instance as in Plate XV. 



Here (Fig. A) is the head. This middle region, 

 here separated into its constituent rings, (Figs. B, 

 C, D) will correspond with the ehest; and here, 



[PLATE XV 



posteriorly, a portion ot tue body, scarcely larger 

 than the head and thorax together, though com- 

 posed of twice as many rings, corresponding to the 

 abdomen. In the imperfect larva (Plate XI, fig. 

 E) we have precisely the reversed proportions in 

 the size of the rings of the different regions of the 

 body, or what will finally constitute these differ- 

 ent regions. The posterior rings in this case are 

 reduced considerably in the perfect condition, but 

 the rings giving rise to the thorax are enlarged, 

 and closely united in fewer joints, so that there is 

 a real reduction of rings, and a real reduction of 

 the moveable parts, inasmuch as the three rings of 

 the chest, which in the earlier stages are equally 

 moveable upon each other, now are united togeth- 

 er, and form only one mass. The reduction, there- 

 fore, of the number of rings or their closer com- 

 bination, or the reduction in size of the posterior 

 ones, with a proportional increase of the anterior 

 ones, when they acquire a higher development, 

 are stages of growth which indicate a progress a 

 really progressive development. 



From these first superficial investigations, we 

 learn one important fact in Entomology that elon- 

 gated species, in any given type, consisting of well 

 divided, uniformly moveable rings, must be con" 

 sidered as lower than those in which the rings 

 combine or unite together, and divide into distinct 

 regions. So that the Caterpillars give us the first 

 hint towards a classification, namely, that Insects, 

 or Articulata at large, stand higher or lower, inas- 

 much as the rings are more or less numerous or 

 reduced, uniformly moveable or combined, uncon- 

 nected, or united into distinct regions. 

 Plate IV, Lecture 6.1 



