INSECTA. 
337 
doubled transversely. Those of others are folded or plaited longi- 
tudinally like a fan. Sometimes they are horizontal, and sometimes 
inclined in the manner of a roof : in several they cross on the back, 
and in others they are distant *. Directly under them, in the Diptera 
are two small moveable threads with a claviform termination, which, 
according to the general opinion f , seem to replace the two wings 
that are wanting. They are called (balanciers) halteres. Other 
two-winged and more extraordinary Insects have also two halteres, 
but situated at the anterior extremity of the thorax, which to distin- 
guish from the others we will cdllprohalteres. Above these append- 
ages is a little membranous scale formed of two pieces united by one 
of their edges and resembling a bivalve shell — it is the alula or 
cueilleron. The same appendage is also observed under the elytra (at 
their base) of some aquatic Coleoptera. 
Many Insects, such as the Melolonthae, Cantharides, &c., in lieu of 
the two superior or anterior wings, are furnished with two species of 
scales, more or less solid and opaque, which open and close, and be- 
neath which, when at rest, the wings are transversely folded. These 
scales or Aving cases are called elytra The Insects provided* with 
them are named Coleoptera, and in such they are never absent, 
though this is sometimes the case with respect to the Avings. In 
other Insects the extremity of the scale is completely membranous, or 
like the Aving ; they are styled Hemiptera. 
The scutel or scutellum is usually a small triangular piece, situated 
on the back of the mesothorax, and betAveen the insertions of the 
elytra or of the Avings. Sometimes it is A’ery large, aad then it 
covers the greater part of the superior portion of the abdomen. 
In various Hymenoptera, behind the scutellum and on the meta- 
thorax, Ave find a little space called the post-scutellum. 
'Idle ambulatory organs of locomotion consist of a coxa formed of 
tAVo pieces, a /emu?’, an uniarticulated tibia, and of a tarsus, Avhich is 
divided into seA'eral phalanges. The number of its articulations 
varies from three to five, a difference Avhich greatly depends upon 
the proportional changes experienced by the first and penultimate 
* The Insect is supposed to be at rest. The rapid vibration of these origans 
appears to us to be one of the principal causes of the humming produced by these 
animals. The explanations hitherto given of it are not satisfactory. 
t They are, in my opinion, appendages of the trachese of the first abdominal 
segment, and correspond to that space, perforated with a small hole, adjacent to 
the anterior side of an opening, with a membranous and internal diaphragm, that 
is seen on each side in the same segment in several species of Acrydium. See my 
M^m. sur les Append. Artie, des Insect., in the M^m. du ^lus. d’llist. Nat. 
X For their cheraicai composition, see Odier, Mem. cit. in the Mem. de la Soc. 
d’Hist. Nat. ; and the article Inseclcs of the same work. 
VOL. I. 
z 
