ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 55 



Monograph on Dragonflies.* — R. J. Tillyard has provided an 

 admirable monograph on Odonata, correlating morphological, phylo- 

 genetic, and physiological data, and including numerous personal 

 •observations. It is in every way a first-class piece of work. The 

 Odonata form a.singularly isolated group, marked by high specializations 

 of structure, superimposed upon an exceedingly archaic foundation. No 

 near relatives exist to-day, nor since Palaeozoic times. The Plectoptera 

 or May- Flies are nearest to the Odonata, but the affinity is slight. 

 The Protodonata of the Upper Carboniferous were the ancestors of the 

 present-day Odonata, but an unfortunate gap in the Trias hides the 

 €xact line of descent from our view. 



The leading characters of the order are summarized ; a detailed 

 account is given of the external features of adult and larva. In the 

 male of Hemiphlebia mirabiUs the anal appendages of the male are 

 rather long, white, and very conspicuous. The inferior pair are like 

 white ribbons. The male uses them to attract the attention of the 

 female by waving them about while at rest on a reed stem. The female 

 replies to his signals by showing the whitened underside of the tip of 

 her abdomen. 



A full account is given of the general structure, histology and 

 development of the wings. The wing-bud is an ectodermic evagination, 

 in the form of a small bag lined internally with hypoderm cells and 

 ■externally with the cuticle. Between the layers of hypoderm is a 

 narrow prolongation of the hsemocoele, filled with blood. Into this 

 space the tracheae soon penetrate and fine nerves accompany them. At 

 the metamorphosis the wings appear as crumpled bags ; blood is forced 

 in and the bag expands, is smoothed out, and dries. The fluid persists 

 for an hour or two or for several days, according to the weather and 

 other factors, and gives the wings a pale greenish colour, while the 

 additional refraction of the rays of light through the still separated 

 media gives a beautiful iridescence to the surface. 



The author gives an account of the macroscopical and microscopical 

 structure of the various systems. Various authors have stated that the 

 mushroom bodies of the dragonfly's brain are rudimentary, and that the 

 intelligence is of a low order, but the fact is that the mushroom bodies 

 -are very well developed, though of a generalized type, and the assump- 

 tion that they are the sole seat of intelligence is unwarranted. The 

 movements of feeding are almost certainly controlled from the sub- 

 <ESophageal ganglion, and many facts show that the ventral ganglia 

 have much independence. The vision is probably the keenest among 

 invertebrates, and its range for the detection of movements may be 

 ten to twenty yards. A dragonfly sees objects best at distances from a 

 few inches to five or six feet, but this is long-sighted compared with 

 other insects. The number of facets in the compound eye ranges from 

 about 10,000 in the smaller Zygoptera to 28,000 or more in the largest 

 ^Eschninas. The so-called "internal light" of a dragonfly's eye, which 

 is beautifully coloured, is a reflection from the interior. The compound 

 •eyes are large and functional in the newly-hatched larva ; the ocelli 



* The Biology of Dragonflies. Cambridge : 1917, xii and 396 pp. (4 pis. and 

 188 figs.). 



