OF THE ANTENNA OF INSECTS. 391 



They exist on the last nine segments more towards the distal end of each. The different 

 forms are shown at N. fig. 3 a a, fig. 4. : the diameter of the closing-in membrane is about 

 -PTQ-iS inch ; the length of the sac is from xqVo to ttttjt inch. 



But, besides, there is another form of what seems to be the same structure, and whicli 

 has a rather less-marked parallel in the antenna of Fronoeua inetahilis (to be described 

 next). There will be observed at N. fig. 1 b b b, a number of small closing-in membranes, 

 of a diameter of 4^50 ™ch ; behind each is a very small sac, leading to a long delicate 

 tube, which, bending towards the base, dilates into an elongated sac having its end in- 

 verted, as may be also seen in the larger sacs (see N. figs. 2 & 3 i). What tlieu' specific 

 use may be, it is at present impossible to say ; but, supposing these organs to be auditory, 

 we may easily conjecture that they would be able to appreciate notes in a higher key. 

 The nerve in this antenna is well seen, throwing off branches to the organs in its course 

 upwards. 



The antenna of Pronoeus metabilis consists of thu-teen segments, and possesses three, 

 if not four forms of these organs, disposed in groups on one side of each segment : — 



First. In the middle of the segment are a number of small closing-in membranes, 

 of the diameter of 4-300 i^ch ; they are raised above the level of the surface, and have a 

 small elongated cavity behind them, which is shown at O. fig. \dd, fig. 3 d. 



Secondly. On the side are level, circular or slightly oval closing-in membranes, with 

 an oval opening just beneath, leading into sacs ; in the centre of each inner siu:face is 

 a papilla pointing inwards. These are shown at 0. fig. \bb, fig. 3 b. The diameter is 

 ■53 00 men. 



Thirdly. There is a scanty group of organs towards the middle of the segment, whose 

 closing-in membrane is also level and quite circular ; and beneath it is a shallow ca\ity, 

 having a small opening in the centre leading into a tube, which soon dilates, and is con- 

 nected with, a curved tube, which expands in the form of a trumpet. The exact method by 

 which these trumpet-shaped expansions end internally is difficult to decide, owing to the 

 delicacy of the parts and the thiclmess of the antenna-wall. It is impossible to make out 

 the exact nature of the junction at the curve. This form I have shown at O. fig. 1 c c, 

 fig. 3 c c. There is a strong resemblance between these and the long expanding tubules 

 in 3fyrmic(( ; and they probably have the same function. 



Fourthly. On the remaining portion of the half-antennal sm'face occupied by these 

 structures, are a number of low projections, at first sight like dwarf hairs ; but on closer 

 inspection they will be found to be conical projections of a delicate membrane protruded 

 from the centre of a depression of the cuticle, and having a sac behind : to these the 

 nerve can be plainly seen passing, as in the other forms ; and a quantity of firm granular 

 matter may be observed at the j\mction of the nerve with the sacs. There is no real 

 analogy between these organs and true hairs. Their diameter is xToo inch, and they are 

 shown at O. fig. 1 a a, fig. 2. 



That these different forms ai-e capable of imparting to the insect the power of appre- 

 ciating notes of different pitch, wiU scarcely be denied, if we admit that they are auditory 

 organs; while, if considered as olfactory, it would be difficult to explain the reason 

 of such a varietv of foi'nis. 



