104 AN INTRODUCTION TO ENTOMOLOGY 



cephalad and the paired openings become a single one. This is the 

 reason that in the adult there are no spiracles in the prothorax. 



The Salivary glands. — The term saUvary glands is a general one, 

 applied to various glands opening in the vicinity of the mouth. The 

 number of these varies greatly in different insects; the maximum 

 number is found in the Hymenoptera. In the adult worker honey- 

 bee, for example, there are four pairs of glands opening into the 

 mouth; three of these are represented in Figure 120 and the fourth 

 in Figure 121. These are designated as the supracerebral glands 

 (Fig. 120, j), the postcerebral glands (Fig. 120, 2), the thoracic 

 glands (Fig. 120, 3), and the mandibulary glands (Fig. 121), 

 respectively. 



II. THE MUSCLES 



There exist in insects a wonderfully large number of muscles; 

 some of these move the segments of the body, others move the appen- 

 dages of the body, and still others are found in the viscera. Those 

 of the viscera are described later in the accounts of the organs in 

 which they occur. 



The muscles that move the segments of the body form several 

 layers just within the body-wall, to which thsy are attached. The 

 inner layer of these is well shown in Figure 122, which is a copy of 

 one of the plates in the great work by Lyonet (1762) on the anatomy 

 of a caterpillar, Cossus ligniperda. The two figures on this plate 

 represent two larvae which have been split open lengthwise, one on the 

 middle line of the back (Fig. 5), and one on the middle line of the 

 ventral surface (Fig. 4) ; in each case the alimentary canal has been 

 removed, so that only those organs that are attached quite closely to 

 the body-wall are left. The bands of parallel fibers are the muscles 

 that move the segments. It should be borne in mind, however, that 

 only a single layer of muscles is represented in these figures, the layer 

 that would be seen if a caterpillar were opened in the way indicated. 

 When these muscles are cut away many other muscles are found 

 extending obliquely in various directions between these muscles and 

 the body-wall. 



In the head and thorax of adult insects the arrangement of the 

 muscles is even more complicated ; for here the muscles that move the 

 appendages add to the complexity of the muscular system. 



As a rule, the muscles of insects are composed of many distinct 

 fibers, which are not enclosed in tendinous sheaths as with Verte- 



