354 



Microscopical Essays. 



which are generally turned in a fpiral form, and come fo neat 

 each other as fcarce to leave any interval; thefe threads are 

 curioufly united with the membrane which occupies the intervals, 

 and form a tube which is always open, notwithftanding the 

 flexure of the veffel. There are alfo. many other peculiarities in 

 it's firuclure, which cannot be well explained without more plates. 



The principal tracheal vefiels branch out into 236 fmaller ones,, 

 from which there fpring 1326 different ramifications. 



The part of the caterpillar which naturalifts call the heart,, 

 without being certain that it performs the functions thereof, is 

 of a nature very different from that of larger animals. It is. 

 almoft as long as the caterpillar itfelf, lies immediately under the 

 {kin at the top of the back, entering into the head, and ter- 

 minating near the mouth. It is large and fpacious towards the 

 laft rings of the body, and diminiflies very much as it approaches 

 the head, from the fourth to the twelfth divifion ; it has on both 

 fides, at each divifion, an appendage, which partly covers the 

 mufcles of the back ; but growing narrower as it approaches the 

 lateral line, forms a number of irregular lozenge-fhaped bodies. 



This mufcular tube has been called the heart of the caterpillar ; 

 firft, becaufe it is generally filled with a kind of lymph, which 

 has been fuppofed to be the blood of the caterpillar ; fecondly, 

 becaufe in all caterpillars, whofe fkin is in fome degree tranf- 

 parent, continual, regular, and alternate dilations and contrac- 

 tions may be perceived along the fuperior line, beginning at the 

 eleventh ring, and going on from ring to ring to the fourth, 

 whence this veffel has been confidered as a file of hearts; but 



