874 



INSECTA. 



any surface in walking these hooks attach 

 themselves, and are released again when the 

 sole of the foot is contracted, previously to the 

 caterpillar's raising it to make another step 

 forwards. In the Sphinguhr the abdominal 

 feet are formed of two parts, the external one, 

 broad, semicircular, and edged with minute 

 hooks, directed inwards like a claw, and the 

 internal one smaller, with its hooks directed 

 outwards, so that two parts of the foot are 

 opposed to each other, and grasp the surface 

 upon which they are walking like the foot of a 

 bird. It is with these that the Sphinx at- 

 taches itself so firmly to the stems and branches 

 of plants, that it is often almost impossible to 

 remove it without injury. In the Sphinx there 

 are four pairs of these legs, attached to the 

 seventh, eighth, ninth, and tenth segments, 

 besides one pair at the thirteenth, or anal ex- 

 tremity. In some Dipterous larvae the abdomi- 

 nal legs are the only organs of locomotion as 

 in the rat-tailed larva of Eristalis tenax. 



In every instance these abdominal legs are 

 only processes of the exterior covering of the 

 insect, furnished externally with peculiar deve- 

 lopments of the cuticle, in the form of hardened 

 spines or hooks like the claws and nails of ver- 

 tebrated animals, and internally with a greater 

 development of certain portions of the muscles 

 of the abdomen. We have full proof of this 

 in those numerous apodal larvae which are 

 capable of locomotion, as in most of the 

 Muscidie, the common maggots. In all these, 

 in which both the true and false legs are entirely 

 absent, the whole external surface of the body 

 is modified for this purpose. In the masgot 

 of the flesh-fly the whole anterior part of every 

 segment is surrounded and beset with numbers 

 of very minute hooks, with their apices directed 

 backwards. With these the larva attaches 

 itself to the surface over which it moves, and 

 carries itself along by the alternate contraction 

 and relaxation of the longitudinal muscles of 

 its body. A beautiful adaptation of these 

 dermal hooks to the peculiar habits of the 

 individual is observed on comparing their form 

 and position on the bodies of the larvae of two 

 very distinct species of (Estrus, the one (Estrus 

 ovis, parasitic in the head of the sheep, the 

 other beneath the skin on the backs of oxen, 

 (Estrus bovis. In the first of these larvae, which 

 moves about freely in its habitation, the hooks 

 (Jig. 360) are all directed backwards around 

 the posterior margin of each segment, a direc- 

 tion rendered necessary for their employment 

 as organs of locomotion ; but in the latter insect, 

 which is confined to one spot for many months, 

 in the tumour occasioned by it on the back of 

 the ox in the cellular tissue beneath the skin, 

 the hooks are not required as organs of pro- 

 gression, but yet are rendered necessary for the 

 purpose of retaining the larva in its nidus un- 

 affected by the varied muscular movements of 

 the parts around it. To accomplish this object 

 each segment of the larva is provided with two 

 sets of hooks. One of these is arranged around 

 the anterior part of the segments, and consists 

 of very numerous minute sharp-pointed spines, 

 directed forwards, while the other is composed 



of strong flattened scales with curved points, 

 very much larger but less numerous than the 

 preceding. These are disposed around the 

 posterior part of the segments, and have their 

 points directed backwards. The effect of the 

 spines thus placed in opposite directions evi- 

 dently is that of retaining the larva in exactly 

 the same position among the cellular tissue in 

 the back of the animal, while the greater 

 strength of the posterior spines enables it at 

 will to penetrate deeper beneath the skin of its 

 victim. 



We have thus seen that in apodal larvae en- 

 dowed with powers of locomotion the place of 

 the true organs of progression is supplied by 

 peculiar developments of the cuticular covering 

 of the body, analogous to the scales on the 

 bodies of Ophidian Reptiles, and these are 

 employed by the larvae in all their progressive 

 movements in the same manner as the scales 

 on the body of the snake. But in those apodal 

 larvae which remain in the same locality until 

 they have passed through all their changes, as 

 the larvae of the bee and wasp, these develop- 

 ments of the cuticular surface do not exist, but 

 the body is perfectly smooth. 



It is not always, however, that the spines 

 on the bodies of larvae are employed as organs 

 of locomotion since they exist on many larvae 

 which possess both true and false feet, and are 

 then either merely ornamental appendages or a 

 means of defence. But whatever be their use 

 in the economy of the larva, they are only 

 developments of its external covering, and 

 generally disappear when the insect undergoes 

 its change into the pupa state, being thrown off 

 with the skin. 



Growth and changes of the larva. The life 

 of an insect that undergoes a true metamor- 

 phosis is one continued series of changes from 

 the period of its leaving the egg to that of its 

 assuming the perfect state. These are not 

 merely from the larva to the pupa and from 

 that to the perfect animal, during which the 

 insect gradually acquires new organs, but con- 

 sist also of repeated sheddings of its skin, which 

 occur at certain intervals before the larva has 

 attained its full size. These changes and the 

 circumstances connected with them have been 

 more particularly watched in Lepidopterous 

 insects, and have been carefully noted by many 

 naturalists, especially by those of the last cen- 

 tury, Redi, Malpighi, Goedart, Merian, Ray, 

 Swammerdam, Reaumur, Lyonet, Bonnet, De 

 Geer, and others, who concur in their state- 

 ments respecting the manner in which these 

 changes are effected. 



Almost immediately after the insect is 

 liberated from the egg it begins to feed with 

 avidity, and increases much in size. Accord- 

 ing to the observations of Count Uandalo* the 

 common silk-worm, Liparis morl, does not 

 then weigh more than one hundredth of a grain, 

 and is scarcely a line in length, but at the 

 expiration of about thirty days, when it has 

 done feeding and has acquired its full size, its 



* Count Dandalo on Silk-worms (Eng. Trans.) 

 p. 326. 



