AIR-TUBES OF THE LOCUST. 
191 
Trachea'. — The tracheae of insects have long attracted the attention of 
comparative anatomists, and the cnrious spiral thread which lies within 
their interior has been frequently mentioned both by the older as well 
as the more recent writers. Those who wish to become acquainted with 
the opinions of the authors of the beginning of this century, will find a 
capital summary in Shuckard's Burmeister, p. 170. It may, also, be 
well to state that the trachea) do not consist of an "external serous and 
internal mucous membrane," as quoted by Dr. Packard on pp. 40-41 of 
his invaluable "Guide to the Study of Insects," that idea of their struc- 
ture being now known to be incorrect. The true structure of these in- 
teresting air tubes was not known until 1875, when Dr. Chun, one of 
Leuckart's pupils, published an article 242 on the " Rectaldriisen der In- 
secten" in which he incidentally describes with approximate exactitude 
Fig. 6. — Testis of Anabrus, showing the ramifications of the trachea). 
the structure of the tracheae. Leydig 243 had previously found that the 
inner membrane consists of two layers, and that the spiral filaments are 
not distinct and separate, but, on the contrary, intimately connected with 
the inner membrane. Leydig also found the tracheae to have an outer 
layer, which contained nuclei, and which he wrongly supposed to be con- 
nective tissue, even venturing to say that no one could think of regard- 
ing it as an epithelium. Chun, in his paper above cited, was the first to 
show that Leydig was in error in making this statement, and that, in a 
variety of insects, the cellular matrix, which secretes the inner membrane 
and the spiral thread, is really an epithelium. At the same time I made 
similar observations on various insects, particularly on the large water 
242 Chun : TJeber deu Ban , die Entwickelung und physiologisehe Bedeutung der Rectaldriisen bei deii 
Insekten. Abh. Senkberg. Natforsch. Ges. Frankfurt, 1876, Bd. x, p. 27. Structur der Tracheen, p. 39. 
'"Leydig: Arch. f. Anat. u. Physiol., 1855, p. 458. Lehrbuch der Histologie, p. 386. Vom Bau des 
Thierischen Korpers, p. 41. 
