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THE AMERICAN NATURALIST [Vol. XLII 



shoots had an average length of from lf-2 feet, and, 

 issuing within a space of 3 inches from the atrophied tip, 

 gave the dried plant a peculiar broom-like appearance. 

 Upon them, usually immediately at the base, frequently 

 within 2 or 3 inches from the base and sometimes at a 

 distance of one foot from the base of the side shoot, oc- 

 curred elongated swellings, from ^-f inch long and in- 

 creasing the thickness of the stem to three times its nor- 

 mal size. These also occurred on stems of the second type, 

 which bore fasciations. Each of the swellings contained 

 a single orange-colored larva, which Dr. M. T. Cook kindly 

 determined as that of Cecidomyia origeroni. The species 

 of Diptera, to which Cecidomyia belongs, lay their eggs 

 on the surface of the plant, and the larvae, after hatching, 

 penetrate the tissues. In this they agree with the A i nch- 

 nida and Hemiptera. The Hymenoptera puncture the tis- 

 sues and deposit their eggs within the plant tissues. It 

 has long been a question in exactly what manner the ab- 

 normal growth due to gall insects is caused. Some as- 

 cribe it to mere mechanical irritation on the part of the 

 larvae, others believe it to be due to a chemical stimulus 

 emanating either from the parent insect, which, at least in 

 some instances, deposits, along with the egg, a certain 

 chemical substance, or from the young larva? only. 48 The 

 latter happens in the case of the gall caused by Cecidomyia 

 Pocb upon Poa nemoralis, 49 and which brings about the 

 formation of roots in places where normally they are 

 never found. But when Nematus Caprecc makes a wound 

 in the leaf tissue for the purpose of depositing an egg, a 

 gall develops, whether an egg is laid or not. Even when 

 the former has taken place, though the egg be subse- 

 quently destroyed, the gall develops just the same, though 

 never attaining full size. For that matter, mere mechan- 

 ical irritation, i. e., the killing of one or a few cells at the 



"Beyerinck, M. W. Beobachtungen iiber die ersten Entwickelungs- 

 phasen einiger ( ynipiclengallen, 177. V eroff entlicht d. d. 1c. Acad. d. Wiss. 



"Beyerinck, M. W. Die Galle von Cecidomyia Fow an Poa nemoralis. 

 Bot. Zeit., 43 20 : 304, 1885. 



