in Three Dimensions (Cambridge: Cambridge University Press) (1997). 

[3] D. Helbing, I. Farkas and T. Vicsek, Nature 407 , 487 (200). 

[4] D. Helbing, I. Farkas and T. Vicsek, Phys. Rev. Lett. 84 , 1240 (2000). 

[5] Physics Today 60 , 28 (2007); C. Feare, The Starlings (Oxford: Oxford University Press) (1984). 

[6] S. Hubbard, P. Babak, S. Sigurdsson and K. Magnusson, Ecol. Model. 174 , 359 (2004). 

[7] E. Rauch, M. Millonas and D. Chialvo, Phys. Lett. A 207 , 185 (1995). 

[8] E. Ben-Jacob, I. Cohen, O. Shochet, A. Czir´ ok and T. Vicsek, Phys. Rev. Lett. 75 , 2899 (1995). 

[9] Y. Harada, A. Nogushi , A. Kishino and T. Yanagida, Nature (London) 326 , 805 (1987). 

[10] F. J. N´ed´elec, T. Surrey, A. C. Maggs and S. Leibler, Nature 389 , 305 (1997). 

[11] Volker Schaller and Andreas Bausch, private communi- cation. 

[12] T. Vicsek et al. , Phys. Rev. Lett. 75 , 1226 (1995); A. Czirok, H. E. Stanley, and T. Vicsek, J. Phys. A 30 , 1375 (1997). 

[13] G. Gr´egoire and H. Chat´e, Phys. Rev. Lett. 92 , 025702 (2004). 

[14] M. Aldana, V. Dossetti, C. Huepe, V. M. Kenkre, and Larralde, Phys. Rev. Lett. 98 , 095702 (2007). 

[15] H. Gruler, U. Dewald, M. Eberhardt Eur. Phys. J. B 11 , 187 (1999). 

[16] J. Toner and Y. Tu, Phys. Rev. Lett. 75 , 4326 (1995). 

[17] J. Toner and Y. Tu, Phys. Rev. E 58 , 4828 (1998). 

[18] R. A. Simha and S. Ramaswamy Phys. Rev. Lett., 89 , 058101 (2002). 

[19] K. Kruse, J. F. Joanny, F. J¨ ulicher , J. Prost, and K. Sekimoto, Eur. Phys. J. E 16 , 5 (2005). 

[20] S. Ramaswamy, R. A. Simha and J. Toner, Europhys. Lett. 62 , 196 (2003). 

[21] E. Bertin, M. Droz, and G. Gr´egoire, Phys. Rev. E 74 , 022101 (2006); E. Bertin, M. Droz, and G. Gr´egoire, J. Phys A: Math. Theor. 42 , 445001 (2009). 

[22] S. Mishra and S. Ramaswamy (to be published). 

[23] A. Baskaran and M. C. Marchetti, Phys. Rev. Lett. 101 , 268101 (2008). 

[24] A. Baskaran and M. C. Marchetti, Phys. Rev. E 77 , 011920 (2008). 

[25] A. Baskaran and M. C. Marchetti, J. Stat. Mech (2010)(to appear) 

[26] A. Baskaran and M. C. Marchetti, Proc. Natl. Acad. Sci. USA 106 , 15567 (2009). 

[27] F. Peruani and L.G. Morelli, Phys. Rev. Lett. 99 , 010602 (2007). 

[28] Y. Wu, Y. Jiang, D. Kaiser and M. Alber, PLoS Compu- tational Biology bf 312, e253 (2007). 

[29] Y. Wu, Y. Jiang, D. Kaiser and M. Alber, Proc. Natl. Acad. Sci. USA 106 1222 (2009). 

[30] T. B. Liverpool and M. C. Marchetti, Phys. Rev. Lett. 90 , 138102 (2003). 

[31] V. Narayanan, S. Ramaswamy and N. Menon, Science 317 , 105 (2007); 

[32] R. Welch and D. Kaiser, Proc. Natl. Acad. Sci. USA 98 , 14907 (2001). 

[33] J. Butler and E. S. G. Shaqfeh, J. Fluid Mech., 468 , 205 (2002). 

[34] E. Kuusela, J. M. Lahtinen, and T. Ala-Nissila, Phys. Rev. Lett., 90 , 094502 (2003). 

[35] J. Tailleur and M. E. Cates Phys. Rev. Lett. 100 , 218103 (2008). 

[36] S. Deans, J. Phys. A: Math. Gen. 29 , (1996) L613-617. 

[37] A. J. Bray, Adv. Phys. 43 , 357 (1994). 

[38] D. Das and M. Barma, Phys. Rev. Lett. 85 , 1602 (2000). D. Das, M. Barma and S. N. Majumdar, Phys. Rev. E 64 , 046126 (2001). A. Nagar, M. Barma and S. N. Majumdar, Phys. Rev. Lett. 94 , 240601 (2005); S. Chatterjee and M. Barma, Phys. Rev. E 73 , 011107 (2006). 

[39] S. Mishra, F. Ginelli, H. Chat´e, S. Puri and S. Ra- maswamy (unpublished) 

[40] M. Aldana, H. Larralde and B. V´ azquez, Int. J. Mod. Phys. B 23 , 3459 (2009). 

[41] M. C. Cross and P. C. Hohenberg, Rev. Mod. Phys., 65 , 851 (1993). 

[42] O. A. Igoshin, R. Welch, D. Kaiser and G. Oster, Proc. Natl. Acad. Sci. USA , 101 , 4256 (2004). 

[43] N, Verstraeten, K. Braeken, B. Debkumari, M. Fauvart, J. Fransaer, J. Vermant and J. Michiel, Trends in Micro- biol., 16 , 496 (2008). 

