16 INSECTS NOXIOUS TO AGRICULTUKE. 



of Aphis are seen two erect more or less prominent tubes, called 

 " cornicles " or " nectaries/' and it is the function of these ta 

 excrete the honeydew.* No European entomologist has, it is 

 believed, seen or described the organ of honeydew-excretion in 

 the Coccididae. Some observations by the author of this work 

 in 1886 demonstrate its existence as a cylindrical tube exserted 

 from the ano-genital orifice after the manner of a telescope, the 

 furthest-extended tube being the most slender. This organ, 

 extremely difficult of detection when not in use except in the 

 single genus Coslostoma is at intervals pushed out to its full 

 extent, and at its further extremity there appears a minute 

 globule of yellowish, nearly transparent, glutinous fluid, which 

 rapidly expands like a soap-bubble, and then, suddenly breaking, 

 falls in spray on the leaf beneath. In the second stage of the 

 female of Coslostoma zcelandicum this organ may be detected 

 more easily than in any other Coccid ; but the act of protrusion 

 of the organ and the formation of the drop of honeydew are 

 apparently by no means frequent, and many long observations 

 may be made without witnessing either. f (The organ and the 

 honeydew-drop are shown in Plate xxii.) 



For the purposes of this work further details as to the 

 production of honeydew are not necessary. But as to its effect 

 on plants it is requisite to be more particular, and the attention 

 of tree-growers and gardeners is specially directed to the follow- 

 ing points. It has been said above that when the bubble of 

 honeydew has been expanded to its full size it breaks into spray. 

 Now, as a general rule, Coccids are found almost exclusively on 

 the under sides of leaves (when not on the bark). Some, as 

 Lecanium hesperidum and a few others, may be seen on the 

 upper side ; but the general rule is as here stated. It follows 

 that the spray of honeydew from the burst bubble falls, not on 

 the leaf where the insect is, but 011 the upper sides of the leaves 

 below it. These upper surfaces, being more exposed to light 

 and air than the lower ones, are usually deserted not only by 

 the Coccids but by other insects also, and so there is not much 



* The fluid also emerges from the anal orifice ; but, seemingly, no mention 

 is made by any observer of any special honeydew-organ protuding from the anus 

 of Aphis. 



t Mr. Comstock (" Eeport on Insects," U.S. Dept. of Agric., 1881, p. 22) states 

 that on gently rubbing a Dactylopius two small drops of fluid, which he con- 

 siders to be honeydew, can be seen to emerge from orifices on the dorsal side of 

 the sixth abdominal segment ; but he mentions no special organ in the body 

 The experiment has been tried on Dactylopius in this country without success. 



