248 THE VITAL FUNCTIONS. 



rated, no longer increase in size, and require but 

 little nourishment for the maintenance of life. 

 This, however, is a state not calculated for so 

 long a duration as that in which the develope- 

 ment is advancing ; and accordingly, the period 

 during which the insect remains in the imago 

 condition is generally short, compared to that of 

 the larva, where a large supply of nutriment, and 

 a rapid circulation of the fluids concur in main- 

 taining the vital functions in full activity. Thus 

 the Ephemera, which lives for two or three years 

 in the larva state, generally perishes in the course 

 of a few hours after it has acquired wings, and 

 reached its perfect state of maturity. 



In proportion as the changes of form which 

 the insect undergoes are less considerable, the 

 evidences of a circulation become more distinct. 

 Such is the case in many of the Apterous In- 

 sects, composing- the family of Myriapoda: in 

 the Scolopendra morsitans (Linn.), for instance, 

 Dug^s observed the dorsal vessel dividing into 

 three large branches. 



Most of the tribes belonging to the class of 

 Arachnida have likewise a dorsal vessel very 

 analogous in its structure and situation to that of 

 insects ; and as none of them undergo any meta- 

 morphosis, their vascular system admits of being 

 considerably developed, and becomes a per- 

 manent part of the organization. Fig. 346 

 shows the dorsal vessel of the Aranea domes- 



