IV. INSECTA: HEXAPODA, HYMENOPTERA 



125 



exceeds the ether thoracic somites, so that these, especially the prothorax, 

 seem but parts of the strong mesothorax. Besides, the first abdominal 

 ring unites to the thorax so intimately in the Entophaga and Aculeata as to 

 seem part of it. The constriction which then separates thorax and abdo- 

 men comes between the first and second 

 abdominal somites, and when the second 

 (petiole) is elongate the stalked abdomen, 

 familiar in the wasps, results. 



The sexes are distinguished by the 

 genital armature. The female is pro- 

 vided with the ovipositor already de- 

 scribed (p. 413), which when used for 

 this purpose only (terebra) permanently 

 projects from the hinder end of the body 

 (fig. 474), but when used as a sting 

 (aculeus), can be drawn into the body 

 when at rest. The sting, naturally lack- 

 ing in the male, is connected with a 



FIG. 474. Sirex gigas, saw fly 

 (after Taschenberg). 



poison gland, the secretion of which owes its effect not, as once believed, 

 to formic acid, but to a little known basic substance, possibly secreted in 

 smaller accessory glands. 



The distinction between terebra and aculeus affords characters of system- 

 atic importance; others are furnished by the development, which is always 

 holometabolous. The pupae, in all important points, are similar (pupae liberae), 

 but two kinds of larvae are distinguished. Some have well-developed legs. 

 Others have footless larvae (fig. 60). The first occur where the larva must shift 



for itself, the second where it is surrounded by an abun- 

 dance of food, either provided by the parents or by the 

 host in which it is parasitic. 



Sub Order I. TEREBRAXTIA. Terebra present; 

 larvae with feet at least on the thorax; eggs laid on leaves 

 or in wood, usually without gall formation; the larvae 

 therefore must move in order to feed. TI:XTI:RKDIMI> i , 

 saw flies, feed on leaves, larvae caterpillar-like. Ciinbc.v,* 

 Nematus.* SIRIOID.E (Vroceridie, fig. 474), horn tails, 

 larvae bore in wood. Sub Order II. ENTOPHAGA. 

 Terebra present; larvrc legless, parasitic in galls or in 

 animals. Some use the ovipositor to lay their eggs in 

 plants. Galls are then produced, diseased structures by 

 which the larvae are nourished. Others lay their eggs on 

 or in other insects. The young feed on the host and at 



last cause its death, often before the completion of the metamorphosis. Gall- 

 producing forms are the CYNIPUX-E; some afford examples of heterogony 

 (p. 132), the alternating generations distinguished bv different structure, by 

 sexual and parthenogenetic reproduction, and by different kinds of galls, and 

 have frequently been described as different genera. The un/niliiifs lay their 

 eggs in the galls of other species. The insect parasites are divided among 



FIG. 475. Chalcis 

 ftavipes* (after 

 Howard). 



