374 INSECTS. DIPTEKA. 



a fan. The larvae have a scaly head, live on hymenopterous insects, and are metamor- 

 phosed into a pupa formed by the skin, and preserving its primitive form. This or- 

 der comprehends but two genera, Stylops and Xenos. 



Gen. STYLOPS, Kirby. 



Antennae biarticulated at their base, divided into two elongated, 

 compressed, unequal branches, of which the upper is jointed ; 

 abdomen retractile and fleshy. 



S. melittce, Kirby. Larva inhabits the bodies of some Andrenetcs. 

 Lin. Trans, xi. 112. 



Gen. XENOS, Kirby. 



Antennae triarticulate at the base, and divided into two elongat- 

 ed, slender, semicylindrical, equal branches, without joints. 



X. Rossii, Kirby. Black ; antennae with compressed branches ; tarsi 

 fuscous. Parasitical on the Vespa Gallica. Lin. Trans, xi. 116. 



X. Peckii, Kirby. Blackish fuscous ; antennae with semicircular 

 branches, dotted with white ; tarsi fuscous. Parasitical on the 

 Polistes fuscata of Fabricius, in America Lin. Trans, xi. 116. 



ORDER XI. DIPTERA. 



Six feet ; two membranaceous extended wings, and a balancer un- 

 der each in the greater number ; a sucker composed of a va- 

 riable number of scaly pieces in the form of setae, either in- 

 closed in the upper furrow of a sheath or inarticulated probos- 

 cis, terminated by two lips, or cased in one or two plates. 



The teguments of the body in this order are generally thin and slightly coriaceous. 

 The eyes are large, particularly in the males, and the head in the greater number 

 has three ocelli. The proboscis is formed, first of a univalve sheath, folded above, 

 and leaving between its margins a furrow or canal, terminated by two lips, and when 

 these lips are much prolonged, presenting sometimes two bends or knees, one im- 

 mediately before them, and the other near the base. 2. of an interior sucker com- 

 posed of at least two setae, representing the labrum and labium, of from four to six 

 pieces in others, analogous to the terminal lobe of the jaws and to the mandibles. 

 3. maxillary palpi, but no labial ones. The thorax is formed in the greater num. 

 ber by the intermediate segment or mesothorax, the other segments of the trunk be- 

 ing very short It has on each side two stigmata, but the anterior ones are often 

 imperceptible. The abdomen is attached to the thorax by a portion only of its trans- 

 verse diameter. It is composed of from five to nine apparent rings, and terminates 

 in a point in the females. In those in which the number of segments is small, the 

 last forms often a kind of ovipositor or oviduct. The sexual organs of the males are 

 exterior in many species, accompanied by hooks or forceps, and folded under the 

 belly. The legs are generally long and slender, and terminated in tarsi of five joints, 

 of which the last has two hooks and very often two or three vesicular or membran- 

 ous balls, which assist the insect to crawl in a vertical position upon polished sub- 

 stances. Sir Everard Home, in the Philosophical Transactions for 1816, has describ- 

 ed the mechanism by which these and similarly constructed animals are enabled to 

 walk contrary to the law of gravity. The wings in the Diptera are simply veined, 

 extended, and almost always horizontal, the sides often ciliated at the base. Under 

 these are placed two small moveable bodies, formed of a linear stalk, and terminated 

 by a button or club, which are termed balancers (halteres), but of which the use is 



