962 



INSETA. 



but this opinion is not borne out by the physi- 

 cal condition of these parts, which, in almost 

 every instance, is unadapted for such purpose, 

 being covered by a hard bony exterior. In some 

 instances the extremities of the palpi are covered 

 with a soft bulb-like extremity, while in many 

 others which are known to perceive the quality of 

 food very quickly, they yet have the extremities 

 of the part covered with a hard imperforate 

 structure. 



As to the seat of the organ of smell we are 

 quite as ignorant of it as of that of taste. Cuvier 

 and some others have imagined that the faculty 

 of smelling resides in the mucous lining of the 

 spiracles at the sides of the body, but ex- 

 periment has still left the question undecided. 

 From the few observations we have been able 

 to make on this subject we certainly have 

 been led to conclude that it is confined to 

 some part of the head, and not seated at the 

 different spiracles. 



The development of the brain and nervous 

 cord during the metamorphoses exhibits some 

 of the most curious alterations of form that 

 take place in any structure. These changes 

 were first traced by Herold in the great 

 cabbage-butterfly, Pontia Brassicte, and sub- 

 sequently by the writer of the present article 

 in the privet hawk-moth, Sphinx ligustri* 

 and also in the nettle-butterfly, Vanessa urtica. 

 We have seen from the foregoing details, that 

 during the last period of the larva state, at 

 which time the insect has been most frequently 

 examined, certain changes of form have al- 

 ready taken place in different parts of the 

 cord, so that these changes of structure, which 

 at first appear to be effected so rapidly at a 

 certain period, have been for a long time in 

 progress. We have seen that, besides the la- 

 teral approximation of the cords, the first 

 change consists in an union of the eleventh and 

 twelfth ganglia, the latter one being carried 

 forwards ; and that, although a complete co- 

 alescence of these has sometimes taken place so 

 early as a day or two before the caterpillar casts 

 its last skin, yet even at that period the cere- 

 bral ganglia have scarcely become united above 

 the oesophagus. At a still earlier period, when 

 the larva has not yet cast its third skin, we have 

 found the eleventh ganglia perfectly distinct 

 from the twelfth, with a small intervening por- 

 tion of cord, and the cerebral ganglia scarcely 

 touching each other above the oesophagus, and 

 the distance, or extent of cord, between the 

 fourth and fifth ganglia much greater than at 

 the subsequent period when the insect is pre- 

 paring to change into the pupa state. 



We had commenced our observations on 

 these changes in the nervous system on the 

 larva and pupa of the Sphinx, when it appeared 

 desirable also for various reasons to make 

 similar observations on an insect in which 

 these changes were commenced and completed 

 within a short and known period, and for that 

 purpose selected the commonest of our native 

 insects, the nettle-butterfly, Vanessa urtica, 

 which undergoes its changes within fourteen 

 days. The Sphinx remains in the pupa state 



* Phil. Trans, pt. 2. 1832, 1834. 



during the whole winter, by which we are 

 enabled to compare the same changes in an 

 insect in which they have taken place slowly 

 with those in another in which they have been 

 completed more rapidly, and the extent of 

 development at the completion of both is in- 

 variably found to be the same. In order to 

 observe these changes correctly, a large number 

 of the caterpillars was collected at the period 

 when they have ceased to feed, and are about 

 to suspend themselves to undergo their trans- 

 formation, and the moment was carefully 

 watched both when they suspended them- 

 selves preparatory to undergoing their meta- 

 morphoses, and also when they were in the act 

 of assuming the pupa state. By these means 

 a sufficient number of specimens was obtained, 

 and their periods of transformation accurately 

 known. Previously to commencing these ob- 

 servations on the nettle-butterfly, we had noticed 

 in the pupa of the Sphinx a very singular ap- 

 pearance at the base of each optic nerve, which 

 on close inspection was found to be a dark- 

 coloured membrane of an ovate form, from 

 which is developed the choroid of the future 

 eye. The existence of this spot is exceedingly 

 interesting as illustrating the manner in which 

 the complicated organ of vision in the perfect 

 insect is developed. This spot, which at first 

 appears like a dark gelatiniform deposit, con- 

 sists of five black tubercular elevations, having 

 the appearance of so many parts of a corru- 

 gated membrane, and exists before the larva 

 has changed into a pupa. We have never found 

 it absent in any insect that is about to change, 

 but have not observed it until the insect has 

 ceased to feed. 



Two hours after the larva of Vanessa urticte 

 has suspended itself to undergo its transform- 

 ation, and in which state it remains from six to 

 eight, ten, or even twenty-four hours, according 

 to the strength of the individual and other cir- 

 cumstances, before it throws off its last larva 

 skin a considerable alteration has already taken 

 place in the body of the larva ; the cerebral 

 lobes are still distinct from each other, but are a 

 little altered in form, although not yet enlarged. 

 When viewed from above they exhibit a pear- 

 shaped appearance, the anterior part of the 

 lateral surface of each being elongated to give 

 origin to the antennal and optic nerves. At 

 the base of the latter, even at this early period, 

 the dark rudimentary choroid is very distinct. 

 The sub-03sophageal ganglion is enlarged to 

 nearly twice its original size, and the crura are 

 much enlarged and shortened, as well as the 

 cords that connect the second, third, fourth, and 

 fifth ganglia. The last two are separated only 

 by a short interval. The fifth, sixth, and seventh 

 ganglia are drawn closer together, the cords 

 between them disposed in an irregular ziz-zag 

 manner, and the longitudinal direction of the 

 ganglia is in consequence altered. The ganglia 

 from the seventh to the terminal one remain as 

 in the active larva. 



By unremittingly watching a number of 

 larvae through all their preparatory stages, we 

 are enabled to judge within a very short period 

 when the transformation will take place. A 

 little while before the old skin is thrown off 



