ICHNEUMON-FLIES 553 



were stages in the development of one species. In the earliest 

 stage, when only one millimetre in length and about as thick 

 as a horse-hair, the larva is free in the interior of the cater- 

 pillar's body, and has a small head armed only with a pair of 

 mandibles. There are, in addition to the head, thirteen segments, 

 and the last of these is an elongate tail forming nearly one- 

 half the length of the creature. ISTo trace of tracheae can be 

 discovered. In the second stage the larva is still free, an elon- 

 gate tracheal tube exists, the tail has diminished to half the 

 length, the head has become much larger, and rudimentary 

 antennae of one joint are visible ; possibly stigmata are present 

 at this stage, though they cannot afterwards be detected. In 

 the third stage (Fig. 361, C) the larva is encysted, the head is 

 large, the parts of the mouth are all developed, the tracheal 

 system is extensive, and the caudal termination of the body is quite 

 short ; notwithstanding the extensive development of the tracheal 

 system, no stigmata can be found. In the fourth stage the 

 larva is still encysted, the tail has disappeared, the head and 

 mouth parts are reduced in size and development, and the creature 

 has now the appearance of a normal larva. The changes to pupa 

 and perfect Insect take place within the body of the victim, in 

 some cases, if not usually, after it has undergone its metamor- 

 phosis into a chrysalis. Very little information is extant as to 

 the duration of the various stages, but it appears to be the rule 

 that .only one generation appears annually, though in some eases 

 there are pretty certainly two. 



It is very difficult to observe the act of oviposition ; the 

 Ichneumon-flies usually decline to notice caterpillars with which 

 they are placed in confinement. Eatzeburg thinks they will only 

 attack caterpillars that are in a deficient state of health or vitality. 

 Occasionally we may by a happy chance observe the act in 

 Insects at large, and from the records of observers it may be 

 deduced with tolerable certainty that the sense of sight takes no 

 part in the operation. Eatzeburg relates that he saw a Pimpla 

 alight on a leaf of Rhus and thrust its ovipositor through the 

 leaf. On looking to the under-side of the leaf he found that a 

 cocoon of Bomhyx neiistria was concealed there in such a position 

 that it could not have been seen by the Ichneumon. 



Among the most remarkable of the Ichneumon -flies are the 

 Insects of the genera Bhysscc and Thalessa. These fine Insects 



