1002 INSECTS AND AKACHNIDA 



structure, the large suckers being furnished, like the hairs of the 

 fly's foot, with secreting sacculi, which pour forth fluid through the 

 tubular footstalks that carry the discs, whose adhesion is thus 

 secured ; whilst the small suckers form the connecting link between 

 the larger suckers and the hairs of many beetles, especially Curcu- 

 lionidce. 1 The leg and foot of the Dytiscus, if mounted without 

 compression, furnish a peculiarly beautiful object for the binocular 

 microscope. The feet of caterpillars differ considerably from those 

 of perfect insects. Those of the first three segments, which are 

 afterwards to be replaced by true legs, are furnished with strong 

 horny claws ; but each of those of the other segments, which are 

 termed ' pro-legs,' is composed of a circular series of comparatively 

 slender curved booklets, by which the caterpillar is enabled to cling 

 to the minute roughness of the surface of the leaves, &c., on which 

 it feeds. This structure is well seen in the pro-legs of the common 

 silkworm. 



Stings and Ovipositors. The insects of the order Hymenoptera 

 are all distinguished by the prolongation of the antepenultimate and 



FIG. 746. A, foot of Dytiscus, showing its apparatus of suckers : a, b, large 

 suckers ; c, ordinary suckers. B, one of the ordinary suckers more highly 

 magnified. 



penultimate segments of the abdomen (the eighth and ninth ab- 

 dominal segments of the larva) into a peculiar organ, which in one 

 division of the order is a ' sting,' and in the other is an * ovipositor ' 

 or instrument for the deposition of the eggs, which is usually also 

 provided with the means of boring a hole for their reception. The 

 former group consists of the bees, wasps, ants, &c. ; the latter of the 

 saw-flies, gall-flies, ichneumon-flies, &c. These two sets of instru- 

 ments are not so unlike in structure as they are in function. 2 The 



1 See Mr. Lowne, ' On the so-called Suckers of Dytiscus and the Pulvilli of Insects,' 

 in Monthly Microsc. Journ. v. p. 267. 



2 See Kraepelin, ' Untersuchungen iiber den Bau, Mechanismus und Entwicke- 

 lungsgeschichte der bienenartigen Thiere,' in Zeitschr. f. Wiss. Zool, xxiii. p. 289 ; 

 Dewitz, ' Ueber Bau und Entwickelung des Stachels und der Legescheide,' op. cit. 



